Co-op mode is still a pretty good time here, and will take you and a friend a long time to play through. But again, control issues hamper the experience. To do some of the cooperative moves, you have to crouch, then hold Z and hit left on the D-pad. No wonder Nintendo embraced a revolutionary control configuration with the Wii -- it was sick of this crap, too! I found the long jump difficult to aim and the new persuasion move useless, but the back-to-back climbing is pretty darn cool.
Next come the graphics. Splinter Cell games rely on darkness for the gameplay to work, and the visuals need to convey that. On GameCube, they don't. The entire game looks washed out, and it's nigh impossible to figure out if you're under a cover of darkness or not. I like the different goggle variations, though, and EEV (electronically enhanced vision) really helps you pick out cameras and laser sensors.
I've been bagging on Double Agent for GameCube so much that you'd think I worked at Safeway. Honestly, it's an okay game at heart, but it's hard to see that if you've even caught a glimpse of the other versions. It's obvious that Ubisoft slapped this together like a lazy 5th grader's science fair project. Think of it like this: Blues Brothers is one of my favorite movies; imagine if they removed one of the brothers, made the songs crappier, and phoned in everything else. You'd end up with Blues Brothers 2000, and nobody wants that.