First of all as might be expected, it's not exactly like the tv series.
In particuler, fans of the tv series might be confused by the opening. It's understandable as an introduction to some of the characters but it violates some things that were implied and some things that were stated outright about the background of one of the characters. It's possible that the character just lied about that bit of their background, but some of the lies were self-detrimental so it wouldn't really make any sense. So i spent the first few minutes being rather confused before i decided that yes, this was really actually happening.
As a whole the movie is darker and scarier than the series. Much darker and scarier than most of the episodes. There were lots of jump out and go boo moments that left me wishing i had someone there to huddle against. Of course i'm a wuss and get scared by things like Sixth Sense, so you should take that with a grain of salt.
The camera style seemed very different. Lots of swooping and spinning and really close close-ups which i didn't like very much. It also didn't help that after they'd already scared me a couple times the camera would start moving about and i'd be thinking "oh no! Something else is going to jump out and scare me!"
There was lots of action, which is both good and bad. Action is cool but some of it felt like it had been crowbarred in slightly just because this was a move, and movies are supposed to be exciting.
All in all however, as previosuly stated, i did think the movie was a good thing overall even if it wasn't quite able to reach the same level of excelence as the series. I'd love it if another series somehow materialized, but failing that more movies would be good. So everyone go watch it when it comes out on the 30th! :)