I'm playing through Halo (via 360) in 2p coop. I played a bit of Halo1/2 before and was a bit underwhelmed, but deemed the game at least worth playing through once to see what all the hype was about, and - more importantly - for the awesomeness that is coop.
Since we died last time and found that you don't reload checkpoints in coop, we had to start over a chapter (the Halo chapter, which is the second one). The game having been just 'okay' so far, we opted to try out Legendary mode (the hardest mode).
It actually made a world of difference. Being completely overwhelmed in terms of numbers and a 'not bad' AI that dodges and tries to flanks and flushes you out of hiding spots with grenades is great fun. Coop + vehicles + melee + competent AI + shield system + Legendary mode + unlimited respawning as long as one of us stays alive (which is harder than it sounds) = fun experience. I dare say I've been enjoying this more than HL2, though it's an unfair fight since the latter lacks the crucial element that is coop.
In any case, I get the hype now It's not the deepest experience ever, but it makes for awesomingly fun coop sessions. I'm an Xbot
PS. Puzzle Quest is great. Get it if you have a DS or PSP. Either version is fine.