What type of app were you running? Website or regular app? Also what version of Mono were you using? I have been running my server app for a few months now on 3.x and had some issues. For example, the default threadpool size is too small, the default implementation of FastCGI on Mono is super slow (there are alternatives out), and the GC behaves a bit differently (nursery size and collection schedule is different). I did have to learn more about the gritty details than when using .NET on Windows but you can fine tune most things. Things are running pretty smooth now though I am still trying to figure out how to debug from a GDB core dump.