saw it too, and want to talk about it, so:
First, for Non-Tolkien fanatics ( no spoiler ):
it's an awesome fantasy movie, splendid backgrounds, jaw-dropping CGI ( rah the ents!!! ), lot of action, etc. it's a must see
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For the Tolkien fans:
-I warn you a last time, it's 100% spoiler.
I'm a Tolkien addict myself, but I wasn't disapointed. But many might be, coz if I'm an addict, I'm not a fanatic.
There's a LOT of things that are different from the book, the first movie was much closer to its paper counterpart.
First, it ends sooner, before Frodo and Sam get to Chelob ( don't worry, Chelob WILL be in the third movie, you still have Smeagol's monologues about "her" )
Second, the Helm's Deep battle takes a huge importance in the movie, I guess it last from 1/4 to 1/3 of the movie. the trees that come at the end are replaced by Eomer riders. There's an elven warrior group that comes to help -and introduces a new character, but this one doesn't survive the battle, just like all the other elves who all die. A lot of things are shorten ( The ent council is anecdotic for exemple). I find the Rohirim Golden Castle quite disapointing, just look like a big house, with few gold on it when it's supposed to be a castle completly built in gold ( but in fact that whole Rohan country just looks poor and weak in the movie ). Gandalf horse ( I'll avoid names btw, coz I bet french names ae completly wrong translated in english

), tho introduced like he is in the book, seems nothing special in the movie. You have an added battle at the end, with a Nazgul showing up, and Faramir takes much more time to free Frodo. Eowyn isn't shown much as a female warrior

There's a ( very impressive ) fight with Wargs during the Rohirims journey between the golden castle and Helm's Deep, and Aragorn is left for dead there ( not much point in that, just an excuse to let him have a sight at Saruman's army and talk about it to Theoden ). Gollum talks about his past with Frodo at some point ( btw, Gollum CGI is just amazing, and you get to see him close and very often, just like any real character ).
You get to see the full ent battle at Isengard ( yeah, ents do hurt a damn lot

). You get to see Gandalf fighting the Balrog, but you don't get to see the hawk who brings him back.
And many other scenes are different, can't count them all.
All in all I think the movie was excellent, I didn't mind most of the changes but there's a few things that were not necessary and inelegant. The movie is really focused on action this time, people who don't know and love the books should not find this one long and boring like the first one ( I didn't feel that way, but I heard that quite often ) without betraying the essence of the book. I'm just disapointed Cjelob doesn't show up, but you can bet that she will take an important place in the third movie. I suspected the part after the ring is destroyed would be removed from the movie and Levyathan comfirmed it, so they got to fill the gap, so the battle at the door, Minas Tirith siege and Chelob part will compose most of the movie.
So what I think: the movie is not perfect ( but that couldn't be expected ), but it remains the best movie I've seen for, well, pone year, and lives up to both the first movie and the book.
voila!