I am a gamer of a lesser sort; I only play PC games. Which makes me quite unhappy that The Two Towers (and Fellowship combo) was never released for PC.
However, I love Return of the King. Next to the Spider-Man video game, it's one of the first epic video games I played, and it was fantastic.
I don't believe I recognize the serious deviations from the books/movies, necessarily. Gandalf going to Isengard was more of a flashback than a deviation. Obviously, Aragorn didn't really have to fight the King of the Dead, and obviously Sam didn't have to kill a hundred orcs to get to Frodo, but these are all within the range of expected changes for a fighting video game.
I was driven quite mad from trying to finish certain levels. It took me a long time to defeat the King of the Dead, and I died dozens of times during the escape sequence. I did not figure out until after beating the game that you had to do a "swift attack" to make Gollum unsteady. I had a very hard time defeating that level. The computer-guided Sam did just enough auto-attacks to get me through the first time.
Fortunately, now it's all a breeze for me.
The most valid criticisms I heard on RotK were the lack of exploration and the fact that some of the voices weren't real. For my part, I just can't get over how all those orcs pour RIGHT THROUGH the Black Gate. Why all the trouble to lay siege to Gondor if they can walk through walls?
One of the things I like about the video game is how it shows a snapshot of movie production. For instance, the Witchking's helmet. The film makers realized it looked too much like Sauron's, and might confuse audiences, so they changed to the more open design of the movie. Having to escape the collapse of the Paths of the Dead came more from the extended scene of wading through skulls. And the Watchers from Cirith Ungol, that drives me crazy. They begin to animate the whole sequence (you can actually see the eyes of the statues light up as they turn!) and then they just skip past the whole thing. I don't understand why they did that in the movie either.
My younger brother is just now getting a chance at the video game, so I'm rediscovering certain things about the game. Like, how you can kill one of the Ringwraiths on Top of the Wall and then build up wads of experience points on top of one of the towers.
My favorite levels are The Courtyard (it's basically a big cage match with no other traveling or interference!), The Pelennor Fields (easy if you have Legolas's arrows) The Southern Gate and the Black Gate.