This post will be updated as I go back through my mobile DAWs and pull things out of them. I'm not claiming to be an expert on any of this, but since I've had a little bit of experience using these programs I thought you guys might want a little review of each one. Feel free to add any reviews of your own, for these programs and any other mobile DAWs you might have played around with.
For those of you who don't know (probably everyone), I used to produce music under a different alias, mostly minimal chiptuney remixes of MLP songs. Owing to the fact that I was strapped for cash and I was focusing mainly on my studies, I did all of this using a number of DAWs made for the iPod Touch/iPhone/iPad. Mostly, I used my iPod for these songs, occasionally borrowing my parents' iPad for certain apps, which I'll cover below. So after all this, and a good deal of fiddling, I think I have a reasonable idea of the strengths and downfalls of these apps. I hope by imparting this knowledge I'll save someone some time and/or money <3
Also, it would be greatly appreciated if someone/some people who have used apps I haven't were to contribute; I'll add them to this post, to keep things tidy. A few off the top of my head include Meteor, Auria, Cubasis and Rebirth. Reviews of mobile synths would also be great, and I'll see if I can add some a bit later.
Disclaimer: the DAWs created for iDevices are generally low on functionality, because of limited processing power. Overall, if you're serious about making music, I HIGHLY recommend you purchase a DAW for your computer. Otherwise, these apps are good for recording melodies/beats on the go, or for quick experimenting, or of course if you're looking to start making music with something cheap!
Disclaimer 2: I may be wrong about any and all things I've spoken about here. Please let me know if I've made any mistakes, no matter how small, and I'll fix them. It's been a while since I used these apps seriously, and in the meantime I may have imagined things that these apps did or didn't have, so there is a strong possibility I've accused something of being terrible when it's been fixed in an update, or I've just conjured it out of nowhere. Apologies in advance.

1. GarageBand for iPad
Very very good if you're completely new to production. There are lots of presets and 'smart' instruments to help you find your way around, and the touch interface is very intuitive. Instruments include drums, a guitar, some basic synths, pianos, basses, a string section (which in my opinion sounds the best out of all the strings featured in the various apps), and a sampler.
There are a lot of presets in GarageBand to choose from, ranging from classic leads to pads and FX. They are, however, a little bit limited in tweakability; in my opinion it would be best to simply route an external synth through to GarageBand using the Audiobus app, so that you have more control in terms of sound design. Sampled instruments such as the strings, basses, guitars and pianos are all quite reasonable (from my limited standpoint, anyway).
Smart instruments allow users with limited experience to play chords in various keys and use pre-determined riffs with the guitar instrument. I was perhaps most impressed with the smart strings, which allow you to vary between plucked (pizzicato) and bowed strings, with the bowing becoming smoother (legato) as the slider is rubbed faster. You can also disable or enable certain parts of the ensemble (such as cellos, double basses, violas, 1st and 2nd violins) to customise the sound, which I was quite impressed with.
The sampler takes input from the microphone and records it, allowing you to tweak the sample and use it for FX, or to create new and interesting loops/sounds that can be used in your song. I found myself using the sampler quite a lot when I used this app, especially as back then there wasn't a lot of support for importing external synths and effects.
There is also an amplifier option for an electric guitar, which can be plugged into the iPad. Therefore GarageBand also has scope to be used to capture and record a performance from an electric guitar, feed it through an amp and output the processed sound.
Finally, there's a 'Jam Session' mode where multiple devices can be connected so that a live performance can be carried out. Aside from mucking around with friends, I'm not certain this feature will be regularly used as a serious method to record a band performance (though I could be wrong).
I will talk about sequencing a little more once I play around with it for a while (I haven't touched GarageBand for months).
Still, for its price and target audience, it is very good value.
2. Nanostudio
This DAW is probably the most easily customizable of the DAWs I'm reviewing. It comes with the Eden synthesizer which can be used to make some semi-decent sounds, and comes with a large number of presets to get you started (I think 128 or so?). Sample slicing is an exciting feature that can be triggered via an option in the synth; pressing different keys will result in a different part of the pre-loaded sample being played. Aside from that, you can modulate filters, envelopes, LFOs, customise a set of x-y pads using the patch bay, and add effects to your synth.
The TRG-16 drum pad is also customizable and easy to grasp. Editing is simple and works well. Samples can have effects added onto them, resampling is featured, and programming the drum pads is fairly straightforward.
The sequencer overall is probably the best out of all the sequencers from the apps reviewed here thus far- the workflow is good, it was easy to pick up and beats are helpfully colour-coded to help slot notes into their correct positions.
My main complaint is the difficulty in loading sample banks from the computer to the iPad, requiring a program called Nanosync, special TRG folders and a wifi connection. This is only a small hassle, but to be honest a more straightforward iTunes drag-and-drop might have been more straightforward and perhaps a little faster.
A mixer is part of the package, with a compressor, reverb, delay, wave shaper and chorus/flanger effects able to be used on individual instruments or on the master channel. For me, this is one of the biggest selling features of Nanostudio. It makes everything a lot simpler to work with and further steps the app up into a serious production tool. There is also a free version of Nanostudio for Windows and OSX, so you can import projects to and from your mobile device. I haven't personally used this version, so if anybody can give a better insight into how smooth the interface between the two is, I'd be very grateful

Overall, a highly recommended DAW in terms of flexibility and customizable features. My main suggestion is to use this in conjunction with a variety of other synthesizers available for iOS such as Camel Alchemy Synth Mobile, and to import your own sample banks, as the Eden synth and preloaded drum packs are quite constraining after a while. However, as a standalone app, it is also perfectly functional.
3. FL Studio Mobile
Again, suffers from few options in the way of synth design, instead relying on presets (most of which must be purchased in-app). The presets, however, are probably the best among those of the other apps I'm reviewing, in terms of quality. I'm guessing from their names (Toxic Dubby, Groove Machine Talker, Harmless One Lead etc.) that these were all created in their namesake VSTs and resampled here. There are a bunch of loops too, but I never really played around with them too much. Samples preloaded onto the app are quite decent, including a collection of metallic hits, synth risers, Laurie Webb vocals, and miscellaneous FX (like gunshots, footsteps and such). Transferring files to and from this app via the computer is quite simple and hassle-free, which is a bonus.
There are a bunch of mixing/mastering tools, but they're very basic (3-band EQ, simple limiter, filter with a limited automation functionality, amplifier, reverb, delay) which can all only be placed on the master channel, with options to disable them for certain instruments. I would have liked a mixer like in Nanostudio, but I guess that's a bit of wishful thinking.
Sequencing is okay, nothing really special. Colour coded beats are welcome, but the controls took some getting used to at first when editing sequences (I'm still struggling a little with quantisation

FL Mobile's selling point is that it's a companion app to FL Studio for the PC, so files can be shared between the two program's with relative ease. You can download all the presets you've used/purchased onto FL Studio once you've registered both programs, and mix/master using the computer, or simply change the instruments to something more suitable once you've transferred the file from your device to your computer.
All in all, FL Mobile is pretty functional, if limited. Its best use is as a sketch pad on the go, maybe if you decide to write out a melody in the car on the way to school or on the train to work, but actually composing, mixing and mastering a song on this app is most likely going to be difficult.
4. MusicStudio
This is basically a ripoff of FL Studio Mobile (or maybe it's the other way around, I don't know which one came first). Almost every feature is exactly the same, and the layout is also very similar. Some of the instruments received are different to those in FL Mobile, but it just isn't worth buying this app to get those sounds. There are a bunch of in-app purchases for presets, but again it's not really worth the asking price. The orchestral elements are okay, but the synths are a bit tired and not particularly inspiring.
The major differences between the two apps include the chord function in Music Studio, where there is an option to simply press a button to play a predetermined chord, as well as the number of demo songs on Music Studio, which are amusing, if not at all necessary.
The sequencer differs from FL Mobile slightly in that there is no colour coding to guide note-placing, which gets quite irritating after a couple of hours trying to guess where a note goes for a triplet, for example. It was the first app to include an option to directly load samples into the sequencer, but FL Mobile has caught up to it, and it's lost its advantage in that respect.
Overall, Music Studio is probably not the best option when it comes to mobile DAWs; I would say FL Mobile is a bit better in value, at a similar price. If you have money to burn, though, try it out to listen to the developers' take on dubstep, or to see how much effort they put into recreating Mozart's 'Eine Kleine Nachtmusik'.
