While modern warfare 3 has all the ingredients of a good modern warfare game - C130 gunning, a wide variety of modern weapons, toys like a remote control armored... tank, remote controlled chopper turret gun, there i ls simply not enough of it to go around. I completed the single player campaign through in about 5 hours of play time on 'regular' difficulty on my first attempt. It definately could have been longer and that is really my only major complaint.
Some parts such as driving the underwater personal sled-type-thing were a little annoying as it only moved in one direction - forwards, and very easily got stuck on things, but this is one of just a couple of minor quibbles about the game and once you've passed that part the rest of the mission is good.
You are given the opporunity to use the new american XM-25, though I don't believe you have much flexibility with it, it's pretty much point and shoot like the rest of the guns, but hey - it's in there.
Haven't tried Co-op or MP yet.

While some people say that the single player campaign is short, it is certainly much longer than the original, approximately 3x so. Valve have really outdone themselves on the graphics, creating three distinct settings for today's journey through Aperture Laboratories, all of which are very impressice looking and incredibly detailed.

While I did come across a few 'sticking points', most of the chambers are well designed, and you shouldn't feel the need to beat your head against the wall more often than once every 3 or 4 chapters.

New puzzle elements are introduced on a regular basis, and always just as you think 'OK I have the hang of this last one you gave me'. The old style energy balls have been retired in favour of the new 'Thermal Discouragement Beam' (It's a bigass laser), Faith plates which catapult you through the air, light bridges, three distinct 'Gels' which can be painted on surfaces to change their behaviour, and Excursion funnels which carry you through the air.