I'll just start with this to get it out of the way; this was the first film I've seen in 3D, and I am solidly in the "meh, you can keep that crap off my screen" camp. It was kind of neat for some of the scenes (aerial battles,) but I'm very glad they didn't do any gimmicky crap and the movie will be just as good on my plain ol' "flat" tv when it's out on DVD (and yes, I'll be getting it.) Maybe if they actually considered those of us that wear glasses normally it wouldn't be so bad, but I wish the fad would pass.
As far as the movie itself goes, there were some big differences from the book, but they didn't detract from the story. Just like nearly any book-to-film, it's bound to happen. Thankfully it wasn't a full out abortion like Starship Troopers or something.
The Green Men were pretty much spot on, and the airships were even cooler than I even imagined them in my head reading the books. I do wish they would have explained a bit more about how Barsoomian technology actually works though.
Overall, I'm glad we went to an afternoon matinee, so only ended up paying $1 more than standard ticket price. The movie was good, but the "premium" for 3D almost stopped us from even going. If you're not worried about that, or have the option to see it on a regular screen for the normal price, I'd definitely go for it.