Probably steer clear of this title if you have an iPhone 4S.
My first experience with the game was to have it crash within 5 minutes. Only force quitting the app would let me relaunch it successfully.
When it wasn't crashing, it was very slow. Stuttering FPS made it difficult to play. After dialing back all graphics options and force quitting the app, it was reasonably playable. Still, it would often wedge itself somehow and start stuttering, requiring a force quit. It may have been coincidence, but I seemed to have the most success with my phone fully charged and plugged in to the charge while playing.
Even when it was working and fast enough to be playable, it still crashed frequently.
Finally, after completing the last level of the 3rd world (I think) it crashed and now it crashes on launch every time, even after a force quit.
Obviously the game is fun if I played it 3/4's to completion, but I can't recommend a title that performs so badly.