As been said a million times, the knock offs:
- do NOT run the iphone os
- cheap material
- low capacity battery
- mostly not quad band (if not quad band, does not work with all gsm carriers)
- crappy processor -> slow program response
- no multi touch on most models
- no touch on many models, many require using stylus or buttons
- no wifi
- low resolution camera (often called 2MP, but can be crappy VGA sensor interpolated)
- odd looking
- difficult to use (unintuitive software)
- no itunes support (no smart playlist, etc)
- no warranty
- no support
- no apple app store