I started using C2 because I'm learning Unity but needed a break from learning C# every once in a while. Plus, at my stage, I wanted something to knock out prototypes quicker than I can in Unity (at my skill level anyway).
I've used MMF2 a fair amount and I hate it now. My main gripe is the minute you want to publish to something other than Windows you lose about 80% of the functionality of the program (most objects/classes don't work with the iOS, Flash or Android plugins), plus it's expensive. What other people have said about C2 being what MMF3 should have been is about right.
I prefere C2 to GMS for the time being, if only for cost.
The one thing missing from C2 and MMF2 is that I can't think of any major games that have been made with them. GM and GMS has a few - people have mentioned Spelunky already, but you're forgetting Hotline Miami and (presumably) the upcoming sequel is made with an OLD version of GM!
With the new 2D features in Unity, it will be impossible to beat it for functionality, but for getting things done quickly, C2 is pretty cool.