As unhappy as it makes me, I am glad I didn't get the game on the PC, because I doubt it has splitscreen coop.
I played the singleplayer stuff for half an hour, before hitting up the co-op with my brother. We just completed the first mission, which took us about 90 minutes (mostly because were playing at the highest difficulty, and goofing around a touch).
But there are I think 4 or 5 coop missions, all of which should last about 5 hours, which is pretty good.
The coop stuff is like playing Rainbow 6 Vegas: Splinter Cell edition, in part due to the familiarity of the Unreal Engine again. It is actually quite a bit of fun... and some of it is actually really bad ass.
What I played of the singleplayer felt good, but a little too scripted and a touch overly streamlined. The production values are top notch, though the visuals on the 360 don't feel any better than any other Unreal engine powered console title.
There are a couple of changes, that are immediately noticeable. First of all, the entire screen goes black and white when your character is hidden. It is a design decision that has the pros and cons you'd expect. Obviously it feels pretty dynamic, and improves the way you play... but at the same time it feels a bit gimmicky. Plus, be prepared to play almost the entire game in black and white.
The other is the cover system, which is actually quite awesome because it makes you feel like you are actually working towards moving from cover to cover, rather than simply hitting the jump button while in cover, as you would in games like Gears or Mass Effect 2.
In Conviction you have a lot of choice when looking for cover, and you actually must look for it actively by focusing on the base of potentially shielding objects, rather than hitting the combination of buttons blindly.
As I said, this makes things feel more natural, and you are far less likely to run into frustration.
Overall, I think the game is worth the full price on console, if you are able to find a buddy to do coop with.