This is always said... I'd really like to see his stats for how much distance he travels, and the average speed he runs during the match. He always seems to sprint alot more than anyone else on the pitch, bar Rafael / Hernandez. Plus, as a CM you have him battling for balls alot which takes energy out of it. I'd love to see, Carrick/Fletcher/Cleverley do as much running and battling as Anderson in a match and see how they'd fare on energy to spare. Look at boxers, plod around a ring for 36 minutes and cuddle for half the match after round 3, that's because they're using their bodies, which Anderson does in shielding the ball, or going shoulder to shoulder. It's rare Anderson gets outmuscled the way our other midfielders do. He's also more box-to-box than the others, Carrick seems defensive mainly, Cleverly doesn't seem very defensive at all. Scholes generally sits around the middle third of the pitch, and Giggs generally does the same. Anderson seems to be everywhere.
Also, just because he "looks" knackered, doesn't mean he is.. he might not be fully energized by that point, but I bet he'd still smash most other people with his fitness at that point.
Went on a bit of a rant there

Didn't mean to, but I think the "fitness" argument is getting out of hand and overused for him, sure he might not be the fittest person ever, but he's giving it his all, for the entire time he plays.