Just finished the campaign and, well..
That was awesome. Played through on hard. The level variety was very much appreciated, and it directly translated into gameplay variety too. I don't really want to spoil anything, but there are some batshit insane mechanics that spice up the experience. Huge levels with a good mix of platforming and shooting and Titan action. Satisfying boss battles, creative use of architecture to create platforming setups, secrets that feel satisfying to find. Someone (was it more_badass or nirolak? it was some mod) said that the game almost feels like a Valve game, and that's true. It has that "journey to stop the bad thing" vibe that HL2 has, and some of the more varied mechanics you interact with later on feel almost like the gravity gun. While it took me about 6 hours, I was so engaged that I didn't feel like it was too short. It had several moments where I thought it was going to end, but there was more, and that was good. Overall, it's tied for my favorite single player campaign of the year with DOOM.