Well, I've been doing nothing but playing bootcamp MvM in my free time since its release, and here is my opinion:
It is beautiful. You can customize your loadout to counter whatever enemies are coming next (see: buying explosive resistance for a wave full of Demos), and still maintain the flexibility to really play it however you want. You can even do silly yet practical stuff like maximizing the firing rate on... just about anything, really. It's a really nice game for pissing around with a bunch of buddies.
My one regret is that they didn't add in any form of scaling. Someone disconnects because of the massive load the servers are getting? Tough muffins, you're still getting the full wave. This also pretty much prevents you from doing solo MvM for practice or just because you want to mow down robots without dealing with the queue. Regrettable, but understandable given the limitations that trying to scale it would entail.
Okay so I have one other regret, and that is Mannworks is basically hell to defend on. There's no real chokepoints and no real path the robots have to take - sometimes they'll just take the shortest route to the drop and sometimes they'll mosey around for half an hour. You never know, and since an engineer is generally a vital player in MvM, it gets nigh-impossible to determine where your sentry should go to get the most out of it. If you want to play on a map and get a real challenge, play a challenge wave on Mannworks. If you just want to goof around and watch a Scout break the sound barrier, play pretty much any wave set on Decoy or Coaltown.