Here are my thoughts, fresh from seeing it, without reading anyone else's thoughts (well, mostly...) Spoilers galore!
Wow.
Where's the next one?
Things that I liked... Smeagol/Gollum. The CGI is incredible. I think this is the first CGI character I've seen that had believable facial expressions. It was easy to read his emotions on his face... and the way his internal arguments were shown was very believable and suspenseful. The first time it happened, many in the audience laughed; but when it happened again, near the end of the movie, no one laughed. By then, everyone understood what was happening, and that it was important, not silly.
The Ents were well done; like many, I didn't have a firm visualization of my own regarding what they looked like. I think PJ's visualization is a pretty good one, though.
The Battle of Helm's Deep was technically brilliant. Watching in real time on the big screen, I could not tell where the CGI began and ended.
Now, things that I didn't like (yes, there were some...) I didn't like Arwen's wavering on whether to stay or go. I realize why PJ made that change, but I think it could have been done differently.
The Battle of Helm's Deep seemed oddly... flat. It was missing a triumphal moment; maybe that was because this is the middle film, and Helm's Deep was just the first victory of a much longer war. And, the A-Number-1 thing I really, really didn't like...
The whole side trip to Osgiliath with Faramir was unnecessary, IMHO. More than that, it was a nearly complete savaging of Faramir's character. In the book, Faramir resisted the lure of the ring; in terms of strength of character, he was stronger than his brother. In the movie though, he falls to the lure of the Ring, then after seeing the Nazgul he changes his mind and sends the Ring away. Maybe this shows he has understood the danger of the Ring and realized the wisdom of sending it away, but superficially at least it makes him look weaker than Boromir. He's so wishy-washy couldn't even summon the guts to make a grab for the Ring, but chickens out when faced with the Nazgul. From a strong, certain, and wise character he has been transformed into a weak, uncertain, vacillating character. In my opinion, this is the only real mis-step PJ has made in his adaptation.
Spoilers off! Now I'm going to go back and read what everyone else had to say! :)