I thought Halo was a lot of fun as a co-op game, but I would have lost interest pretty quickly if I'd played it on my own. I do understand why people like it so much: The impressive AI on NPCs, the tweaks to traditional FPS gameplay, the relatively cool plot, and the all-around high quality of the game. Having said that, I agree that the levels were way too repititious (not to mention oversized with no save points) and I thought some of the vehicle controls were unecessarily "original". When it was good it was awesome, and when it was bad it was dreadfully boring.
I think the game would have been significantly better if they had actually made it shorter, condensing it to the best parts.
Actually, I've been thinking about a lot of games lately. Hopefully they're fixing this with Halo II. It does look good, and I'll probably play it just like the first, but Halo II probably won't be the game that makes me buy an XBox.