I think it's a great game for fans of the X series. It expands on a lot of stuff and gives fans a chance to see X environments/characters in a more in-depth light. (Even if the story is kinda bland to a point.) There are twists in the story though, from time to time, so that's at least something.
I agree about the comment regarding the dungeons. They're overly simplistic and really boring to go through. It's basically just a race to the boss while collecting items along the way, no strategy or exploring involved period. I did hear that the GC version is more challenging due to the fact that the random battles pop up more often than in the PS2 version, and that's pretty obvious, even to the point of becoming annoying after a while.
One thing that I think I like most about it is the cutscenes. Some of them are really cool, especially the opening scenes involving X, Zero, and Epsilon. The voice acting doesn't bother me, as it could be much much worse.
But anyway, if you're a fan of the X series or just a blue bomber fan in general, pick it up. If not, rent it.