Saw the Two Towers yesterday, and enjoyed it a lot.
Good
The split personality of Gollum/Smeagol (even more effective in the movie)
The fact that the movie series gives the female characters more character than the books do.
Eowyn ought to be senior at Rohan when her brother cousin dies, her brother is exiled, and her father uncle is incapacitated; the book takes the medieval inheritance rules for granted, the movie sympathizes
As in the book, the bonding among the male characters (Frodo/Sam, Legolas/Gimli) (In the book, the heterosexual relationships weren't credible at all; they're better in the movie)
The New Zealand landscape
The ents (though their part is cut in the movie)
Characters that get unbearably prissy in the book (Aragorn, Frodo) are more bearable in the movie.
The dead marshes ("or Frodo goes down with the dead ones and lights little candles").
Creepy Nazgul (though creepier in the 1st movie).Given the good job with Gollum and the Nazgul, I'm looking forward to seeing what they do with Shelob.
The various elf-props (in the event of a water-landing, your elf-cloak will serve as a floatation device)
The strange and pleasant sensation of remembering events from the book as they happen in the movie
Most important -- the sense of being in another world (though the movie can't recapture the feeling of being fourteen years old, with a boring and subjectively miserable life, transported into a rich and complicated alternate universe.)
Not as good:
The episodic pacing is tougher to make work in a movie (as in the book; cuts between moody Mordor-route scenes; complicated Helm's deep battle scenes, Merry/Pippin sublot scenes.)
As in the book the dialog has its clunky moments. After the first few minutes, though, I got caught up in the story and didn't notice so much.
Sauron/Mordor as Ultimate Evil. As in the book, not credible. The internal struggles of the flawed characters are much more interesting.