I've been very satisfied with the films to date. When I first went to see Philosopher's Stone, I remember being really annoyed that the film wasn't following the book down to a T (for example, the absence of Piers Polkiss from the outset), but I know how unreasonable it was to expect them to include everything now, and they are an absolutely fab collection of films. I love watching the children grow up with each one, it really gives the series so much credibility: even minor characters star in each and every one, such as Seamus Finnigan and Dean Thomas.
The first two are pretty comparable - really nice, cheesey and warming (just like the books). Number 3 was very good owing to a dramatic improvement in all the kiRAB' acting (especially Radcliffe's), but as someone already mentioned they did leave out some important stuff, like not really filling the audience in on the animagus/Marauder's Map saga, which is quite a good part of the book.
Number 4 is my personal favourite, can't really explain why, there was just very little wrong with it (well, except the revealing of Moody to really be Barty Crouch Junior and not explaining how that came to be the case). 5 was good, not the best but they cast the newly-introduced characters very well IMO: Umbridge, Tonks, Kingsley, and Luna in particular were all perfect. Considering the book is probably the weakest of the 7 they really managed to make a good film out of it if you ask me.
I'm looking forward to HBP. It's a great book and there aren't millions of different stories going on at the same time, so it should (hopefully) be an easy-paced film that includes the important parts of the book. Apparently it's supposed to be a lot lighter and funnier (in terms of all the love plots going on), which I think would be a welcome relief from the darkness of the previous two.
My main hang-up is that the girl who plays Ginny, pretty as she is, isn't the "hot stuff" she's made out to be in the book, but as she's been in all the films so far I wouldn't have it any other way!