Thought I might as well post about my experience with Splinter Cell Conviction here. I just finished my first playthrough, though I don't know how long it actually took me. It didn't feel too short or too long, which is just fine. I don't have a problem with games being short as long as it's pretty well packed in there and Conviction definitely is action packed.  I think it's rare nowadays for longer games to not be long just for the sake of being long. I don't want filler. If you wanna give me a 20 hour game that's got little to no filler, that's awesome! But so few games nowadays are like that and I for one am glad Conviction chose to go the other way.
As far as I've seen the co-op is also a lot of fun. I had a friend over on the release day and that was pretty much all we did for 4-5 hours. There's definitely a lot of fun to be had there and I really can't wait to play more of it. While I haven't played much of the game online I'm also looking forward to putting some time into that. 
So that's Conviction from where I'm sitting.