Just finished the game a bit ago. My total playtime was 5 1/2 hrs., which, while short and all, is still a good length for a game that's made for replaying. Some thoughts:
The gameplay is fantastic. The controls are pretty much perfection. You have such precise, incredible control no matter how much shit is happening on screen. I posted earlier today about not being entirely sure how I was supposed to really play the game. As soon as I found out about activating AR via roll or jumping cover, the game went from defensive play to fuck yeah. Also, having a menu-less upgrade system was such a great idea. Bravo to that. It's just too bad the cut scenes frequently interrupted where the gameplay was designed to be nonstop.
The environments were actually pretty cool. Seeing the arching rings above me the entire time was probably the coolest in that regard. Very Halo-esque(though not as cool). The change from day to evening to night was also a nice touch to give things some distinct flavor. There was a really cool segment in Chapter 1 or 2 where you're on a monorail and another train with enemies catches up, except that it's upside down, above you. And then it never happened again. The closest thing was the zero gravity segment, which wasn't really anything special.
Story was...well, not worth talking about it. And hey, I didn't come for the story, but man Burns was annoying with his constant yelling. If he was a likable character then I wouldn't have minded so much. Sound was just okay. The soundtrack was boring except for maybe one or two tracks that stuck out. The beeping of Sam's suit when the power is depleted...so annoying.
Recycling the same boss several times sucks, even if it's not really considered a boss encounter, it still takes as long. I'm just referring to the first boss type. I didn't mind the Unknown fights. Those were a lot more fun and felt shorter. The last two boss fights were a blast. The last was particularly tense. Also, those scorpion-type mid-bosses are serious assholes. Good design; they really make keep you on your toes and moving.
Overall I had a lot of fun and I'm betting hard mode is really where Vanquish shines. It's really great to play a game where the gameplay has been polished to that level. However, it sucks that the rest of the game is so bland in comparison. Personally, that kind of thing keeps me from coming back to some games. Aside from gameplay, I tend to replay certain games because I love the atmosphere and the feel of the world. Vanquish definitely doesn't have that appeal, and I wonder if that's the big reason the game seems to have been overlooked by most people(myself included).