Andy Serkis's performance is great, but it's not the Gollum from the book. He's not supposed to have split personalities. I would argue that the book version is much more complex, because Gollum is genuinely conflicted about whether he should serve Frodo or the Ring; he wants to do the right thing, but centuries of being twisted by evil makes that difficult; he doesn't just have a good and a bad personality.
The Ents were awesome. Bonus points for John Rhys-Davies (the actor who played Gimli) voicing Treebeard. Though I'm not sure how well the movie(s) got across the very slow, plaintive, methodical nature of the Ents. I know they did at least to a degree, but it's been awhile since I've seen Two Towers.
Helm's Deep was a really well-done battle scene…but I'm not sure it really belongs in an adaptation of LotR. The book speeds through the battle fairly quickly, but it takes, like, half an hour at least in the movie. In general, I think the movies put so much emphasis on violence that they lose a lot of the charm, as well as the message, of the book. I'm not a big Roger Ebert fan, but I think a quote from his review of the film sums it up very well:
"The Two Towers" will possibly be more popular than the first film, more of an audience-pleaser, but hasn't Jackson lost the original purpose of the story somewhere along the way? He has taken an enchanting and unique work of literature and retold it in the terms of the modern action picture. If Tolkien had wanted to write about a race of supermen, he would have written a Middle-Earth version of "Conan the Barbarian." But no. He told a tale in which modest little hobbits were the heroes. And now Jackson has steered the story into the action mainstream. To do what he has done in this film must have been awesomely difficult, and he deserves applause, but to remain true to Tolkien would have been more difficult, and braver."
Also, the Elves being there made little sense. There weren't a whole lot of elves left in Middle-Earth at that point, and the ones in Lothlorien would have been preoccupied with preparing to defend their home from Sauron's coming forces.
Since I'm in complaining-mode, I'll point out that the ghosts in the book honored their oath and didn't try to kill Aragorn and company.
The hike through Mordor was good, but not so much the journey there. Frodo as much too loyal to Sam to ever tell him to leave, and he would never have sided with Gollum over Sam. Frodo's relationship with Sam is based on brotherly love, but his relationship with Gollum is based on pity. The movie makes them seem more like equals.
Not to be "that guy" (heh, I probably already am ) but it's Grond, not Gron. It's named for Morgoth's hammer.
I don't remember much in it that was whimsical? What did you have in mind?
tl;dr, the later movies (as well as the first, but not as much) make changes that weaken, or at least fundamentally alter, the story, and focus too much on grandiosity instead of humanity.
P.S. You can probably tell I've spent way too much time thinking about this.