Finished Vanquish, very enjoyable, other than a few higgles that would have improved the experience. I had heard that it ended on a cliffhanger and thankfully that's not really the case, it has the equivalent of a Marvel movie post-credits scene which sets up a possible sequel scenario, but the central conflict that started the game is resolved. I played it all the way on hard which wasn't too difficult other than in a few small areas that didn't have much cover and a couple of boss fights (could never find a very reliable, non-tedious way to kill "Bogey"). Rocket sliding and dodging around from cover to cover is very fluid. On harder difficulties you have to rely a bit too much on cover making it less possible to play more aggressively (I was barely able to make use of the shotgun for example).
A more streamlined weapon upgrade system would have been preferred as juggling weapons to get a version with full ammo so that picking up the partially used one you started with will upgrade it, is needlessly cumbersome. It also would have been nice to have a few ways to customize the mech to more better fit your playstyle, such as the ability to increase defense, or increase boost speed, or reduce stamina consumption for melee, etc.at the expense of other attributes. The mech never really changes or gets any new ability during the course of play.
The ability to activate the "totally not bullet time" system while in cover or standing still (can only activate it when rolling or boosting or it auto-actives if you take too much damage) could have been accomplished by holding X while in cover, then aim to enter bullet time. That became an issue sometimes while playing as when you activate bullet time during a roll its like you are doing it from a crouched position so you can't shoot over railings or if you are in tall grass it obstructs your view.
You can have 3 weapons equipped (of 8 possible) at a time and switch to them at will with the D-pad, the fourth slot is used to toggle between a stun and explosive grenade. I would have preferred a slightly more convoluted method to switch grenade type (such as selecting the weapon you already have equipped with the dpad again) in exchange for having a slot on the dpad for a fourth gun as the EMP stun grenade is ridiculously more useful than the explosive grenade so I almost never switched off it.
The melee button is cool, and it is neat that it changes depending on which weapon you have equipped but using it immediately puts you into full overheat (can't activate bullet time) which leaves you completely defenseless. In almost all situations it is preferable to manually activate bullet time (which slowly depletes stamina gauge) and go for head shots rather than melee. Why give you such a deep melee system when using it has such heavy drawbacks compared to the meager benefits? There is one weapon that consumes ammo instead of overheating when you use it, but it does damage so slowly that if there is any other enemy on the field you are likely to get hit and knocked out of the animation. Melee should have only depleted a portion of your gauge or at the very least, put you less than 100% into overheat.
When you use chapter select to replay an old mission, you lose all of your weapon upgrades. If they wanted to keep the playing field even for people doing score chase / time trial runs then you should have been able to start the mission in either free play (where you keep all your weapon upgrades but your score isn't counted for the leaderboard) or time trial where they are reset to base for the duration of the mission.