When I first read this book, and The Fellowship of the Ring, it was in preparation for seeing the movies. I had never read them before then, and I felt like it would be better to read them first, just so I'd have the books as my first experience with the stories. I wound up losing steam with that challenge before the third movie came out, but I can at least say that I read the first two books before seeing the movies.
A bunch of what I recall from the movie -- Gimli and Legolas keeping score during the Battle of Helm's Deep, for example -- seemed like it couldn't have existed in the book. Imagine my surprise when I realized that it was in the books first! That kind of competition seemed like something tied in to the video game audience Jackson was pandering to, so to see it in the book was a bit of a shock. I will say that Tolkien handled that exchange a lot better than I recall it from the movie, so there's that to consider. It felt more like two friends bandying about during the heat of the battle, instead of the one-upmanship that the movie took.
Interestingly enough, the characters who I feel have the most life in the stories so far are Gimli and Sam, two characters whose honor carries them above all else, and whose personalities turn them into comic relief for a good part of the story. I think part of what makes them so memorable is that they aren't so serious all the time. Gimli certainly presents himself that way, but his lust for battle makes him a little more humorous than the rest of the cast. That aspect of his character, I think, was well translated to the movie.
The story suffers mildly for being the second in a trilogy that was never intended to be a trilogy at all, but it's hard to fault the story for that point. Tolkien wanted the books to be released all together as one volume (and, interestingly enough, the printings that I'm reading number the pages of volumes two and three consecutively from volume one), and if The Two Towers is criticized for its lack of exposition and character introduction, then that critic is missing the point. I do find that the story would have been better told interspersing the events of books three and four instead of segregating them, but that's another point for Jackson and his vision.
There's not much to say about the book that hasn't been written before. It's certainly worth reading, though.