MvM is okay. I'm not paying for it, but still. Not bad. Once they restructure the pricing, boost server support, and give it regular tweaks the way they do conventional PvP, it'll make them good money.
There's a lot of crazy shit they'll be able to do under the MvM mode, they can more fully support this growing Mann Co. backstory with some kind of weird campaign/story mode in MvM.