The films would be good if they stopped unnecessarily changing the plot and the sequence of events. Book to film does need some work to make it shorter etc., but most of the stuff changed in HP was complete bollocks and needless.
Plus, the crappy actors. Well, "child" actors. Rupert Grint = good. Dan Radcliffe, Girl-who-plays-Hermione, pretty much everyone else = crap.
3 was awesome because of Sirius, or should I say the magnificence that is Gary Oldman. <3