* Only useful if you live and breath Facebook; it makes email and texts and phone calls second class citizens
* Removes all your widgets
* Would be better as a lock screen than a launcher
* Hides notification bar
* Quick Select only shows Facebook apps (and the most recent app)
It seems like a lot of complaints are from people who didn't really understand what they were installing ("Hey, I like Facebook! Let's try it!"), and were surprised when it turned their phone into a Facebook phone.
A lot of people seem to really like it, too, though.