In iOS 4.0, Apple opened access to the vDSP library on iOS which has some nice DSP functions (FFT, Convolution, etc.). I've been using this library for some time now in my apps.
I've had a few people ask me if it's "easy to use."
It's not too bad and the documentation is there if you know where to look, but initially it can take some time to work out what format the data should go in/out as depending on you're intended use.
Today I've released some of the code I've written to interact with this library at a high level. If you just want to do a straight Real FFT, this may save you some time figuring out the vDSP library.
I plan on adding other features if the demand is there. So if you use it or would like to request features please let me know.
The Code is available in a sample project on Github