My 2 cents: the game is fantastic. I had it preordered for PC and paid full price on Day 1 as I am a big SC fan.
Here's the deal: if you play the game stealthily, plan your moves, and keep replaying sections until you nail the perfect run through an area, it lasts PLENTY long, and you feel like a complete badass. There are a few areas that pretty much require run and gun, but these are far, far, far and few between the stealthy stuff.
Also, IMHO, the campaign is just training for what I consider the meat of the game - deniable ops. I haven't had a chance to try co-op yet, but just the single player sessions are great. Basically you have 7 or so very large levels that you can play in a number of game styles. For the ultimate stealth fan, there is nothing better than infiltration mode - never be seen, ever, not once. Be a ghost. The levels are excellent and well designed, and there is DLC with even more maps to add.
I have probably spent 20 hours dicking around in deniable ops and play it far more than the campaign. The game is just a winner all-around IMO.
It's totally worth the ~$20.
/cents