I'm not a FL Studio user (I use Cakewalk Sonar 8.5 on PC) but I got FL Studio Mobile for my iPhone 3GS because I'm addicted to music apps. I'm a bit disappointed in FL Studio Mobile. There are a lot of audio drop-outs, during recording and playback. Maybe people with an iPhone 4 or iPad will have better luck. I'm not sure if it is a processor issue, but to me it seems like FL Studio Mobile is not efficiently coded for the iOS.
FL Studio Mobile seems to be based on the Music Studio app, which is quite nice and I highly recommend it. Both have the flaw of not being able to add your own samples, and I found FL Studio Mobile's selection of sounds to be limited. Music Studio has a lot of great sounding classical instruments (if you are into that) as well as normal band sounds (bass and electric guitar). Currently I can't get any sounds out of FL Studio Mobile, some sort of glitch...
FL Studio Mobile's keyboard has the nicely playable keys from Music Studio, but the drum pads are super basic. The sequencer functions are adequate, but I found things to be a bit tricky or inconvenient to use. Case in point is that you can not scroll along while you are zoomed in on the piano-roll. You have to zoom out by pressing the magnifier lens icon, move the track, then re-zoom with the magnifier lens: this is tedious for serious editing. One nice feature was the option to clear just the filter modulation, enabling you to save your well-timed playing and discard unsatisfactory use of pitch or modulation wheel (when using the Akai Synthstation 25).
All-in-all, FL Studio is not a bad first attempt, but it is seriously in need of improvement and I am hopeful that future updates will correct some of FL Studio's Problems. However, I strongly recommend Nanostudio over this app. Nanostudio includes the Eden synthesizer which can be tweaked or programmed from scratch to create new sounds, and you can load samples into Eden or for use in the TRG drum pads. Nanostudio offers basic sample editing as well as some nice effects. Sequencing on Nanostudio is very user friendly and quite capable of producing results that are on par with many DeskTop DAWs, and is much easier to use (that means a lower learning curve). Nanostudio really offers a complete mobile music workstation (even better with the Akai Synthstation 25), and all of the files you create on Nanostudio can be uploaded into your desktop DAW later. There is a free PC version of Nanostudio that you can try out available here: http://www.blipinteractive.co.uk/download.php