I'm in about the same boat. Huge fan of the book, and so I was eager - and a bit worried - to see this make the jump to film. Let me say, out of all the offerings out there (audiobook, written book, film) I still think the unabridged AudioBook is probably the best presentation this will ever have.
That said, there are a few things I really liked about this film:
(1) a great introduction and handling of characters, especially flash backs to the first Bugger invasion, which I thought looked fantastic
(2) The scene with Bonzo was incredibly well done, I had worried they'd go soft on this and they didn't.. while it's left up for interpretation in the film, I thought it was pretty clear what happened. It was obvious they couldn't do it exactly as the book for ratings and also because it wouldn't be easy to understand on film, but this was easy to understand and I thought well done.
(3) The battle room was presented in a way that I thought was pretty mindbending.. and it worked. Completely worked. One of the more original, unique effects I've seen in a while.
(4) In a moment not from the book, but I thought great, Mazer explaining that his tattoos were a tribal way to remember his father who was always with him, to allow him to 'speak for the dead'. I thought that was a great tip of the hat for fans of the book and it worked.
Finally, this is where I really appreciated this film: one of my concerns was that they would end fairly quickly after the final beat; I love that the film instead leaves the battle for the confrontation between Ender & Graff, in something that isn't on the page but manages to effectively convey a lot of the internal dialogue of Ender in the book and get across the key points of the book - that how we win is as important as winning itself, and undertaking this may have been wrong from the beginning. I thought it was effective and fantastic.
What I didn't care for:
(1) This film feels very rushed. That's a bit because the book is dense, but damn do we whip through combat school in a hurry.
(2) I wasn't a fan of introducing Bean in the first shuttle launch. It compresses things a bit too much; had he been introduced just a few beats later I thought it would have been more effective.
(3) I disliked them changing Ender's name to Andrew Ender Wiggin. It undoes why his nickname came about; so having the first quotes from AE Wiggin was kind of 'eh'.
The last one is a big spoiler, so..
All in all, this was a crafty presentation that managed to do more then I thought it would/could.