I was waiting for Valve to start it's summer event, (with hopes of them releasing some exclusive items again) and I was beginning to think they had released Pyromania as their only summer event, when it turned out they had something else up their sleeves.
I had never heard of Mann vs Machine until some guy on a trade server just happened to mention it two days before it's official release. Now, instead of just playing again other players, we can face off against hordes of bots.
The real challenge comes from Mann Up mode. While the bot attack is much more intense, the real challenge comes from trying to find team mates who can work together with a good strategy and not quit at the first failed attempt. I've joined many a team with players who would join a class that wasn't really helpful to the effort, or would lose focus from the primary goal (pretty much just stopping the bot carrying the bomb).
There are six levels, and the first one took me 3 days to get through it, but the second one only took a single hour long session. So far, I haven't worked with a team that has been able to overcome the 3 level.