I went with the GameCube in the end... got it about a week ago, and am very pleased with it.
My first go on it was with a few colleagues at work playing Rogue Leader. I was abusing my position and used the conference room with the multimedia projector to play it on. With the lights out and a screen the size of a wall, it's better than watching Star Wars in the Cinema!
Very pleased with it so far, anyway... a little disappointed with the range of games available for the release date, but I'm sure that a lot of really good games will start to appear very soon.
In any case there was no way I was going to go near the X-Box no matter how good the games are for it...
From reports I've heard here in the UK, the GameCube has sold better in Europe in its first week than the X-Box did in the same region. I like to think I've had a hand in that...!