For a basic app, I've found Voice Record Pro to be very good. It has a "High Record Quality" setting and is easy to use. Even with a simple computer mic plugged into the iPhone/iPad, the quality is serviceable for practice review.
(For better quality recordings, I still use the BIAB "record audio" feature, with my guitar plugged straight into the computer tower. I realize that doesn't work for non-electric instruments where a good quality mic would be necessary.)


Cheers! -Alastair