I tried to use MonoGame with two small projects (2D and 3D, in July 2013) and I can say the following:
- My games didn't work on all Windows machines - on some they were just crashing somewhere in videocalls
- The games were not easily portable to other platforms. I tried to build my game for the Mac, but didn't manage to because of Xamarin Studio/MonoDevelop refusing to open/build my project. There were some very obscure recipes on the internet but the whole situation with the platform support made me decide not to bother.
- There is no content pipeline processor in MonoGame. You need to have XNA Game Studio installed to compile your assets, with some additional steps/quirks.
That said, the question arises: why not just use XNA?
So after that I decided to switch to C++ with Gameplay3D or SDL2 for games, and am really really happy with this decision so far.