This is a perfectly functional game, but there are so many little things wrong with it that I found it rather un-fun, and I played it back when it was released.
The plot and characters are awful, and you're forced to deal with them a lot. Even the ridiculous, gravity-defying cleavage of your second-in-command didn't stop me from wishing she'd shut up and go away, and I was a teenager at the time.
In missions the player character feels really awkward: you're always an ungainly tank lumbering through the level. There's no sense of urgency when you're racing against time, and there's no suspense when you're creeping along a deserted corridor. The bewildering number of available weapons mean that you'll never have the right gun for the situation, and you'll spend more time choosing them than using them.
I'm not sure if it would still be the case, but "quick" saving and loading always took forever, and my machine was quite capable of handling the (admittedly pretty) graphics. It's one of many factors that keep you from feeling any immersion in the game.
In the end I just stopped caring, and left it unfinished. It plays (as has already been said) like a glorified tech demo: some interesting ideas and pretty things to see, but no fun.
I have heard good things about XMP - the multiplayer component - but I don't recall it being any more interesting than UT2004. On the other hand, UT2004 is excellent...