I've now finished reading The Hunger Games and I've already seen the movie. I have to say that the movie fares remarkably well and is a very good adaptation. They've had to sacrifice a few things here and there and alter the plot a bit, but over all if you liked the book, you'll probably like the movie.
The main difference between the two mediums, is that the film lacks Katniss' almost constant internal monologue. This is probably to be expected as such things are generally distracting from what's actually going on in the movie. Luckily Jennifer Lawrence makes up for this missing element with some quite solid acting and manages to convey with her body language and facial expressions what Katniss is feeling at the time.
Plot wise, the movie is mostly faithful to the book. They change how Katniss gets the mocking jay pin. Katniss is gifted it by the person she sells her hunting spoils to, gives it to Prim for good luck who then returns it when she says goodbye after the Reaping. I'm not sure whether it was changed because they didn't want to cast another minor part or whether the mayor's daughter just doesn't feature in the later books or what. It certainly doesn't detract from the overall story and adds a bit more tenderness between Katniss and Prim.
There are slight tweaks to a few of the more actiony scenes, like with the wasp things. Rue doesn't make an appearance until the following morning at which point the cutting down scene is condessed into one section. And the dog things attack Peeta and Katniss first, chasing them to the Cornucopia where things play out with Kato.
One thing the movie does that the book doesn't is focus on events that unfold outside the arena once the competition is started. We get to see District 11 react to Katniss' touching display of affection to Rue's lifeless body. Katniss herself is shielded from the trouble she has caused and the end of the film seems to focus more on the love triangle between her, Peeta and Gale. I guess we'll find out in the next movie if this is what they focus on more initially?