I'm happy for them, but I'm not going to risk it. The reward is nice but not nearly worth the risk.

When adopting Java, you can trust it will just work, but with Mono you have to bet it's going to work, you're not going to run into compatibility problems and tooling will be adequate.

Java comes with its own problems, especially in enterprise environments where you might be pinned to a single version. There are plenty of insecure JVMs out there that can't be upgraded on both servers and clients, mainly to ensure compatibility or meet support requirements.

I don't see how it is relevant WRT considering a language/platform for a new project. You're talking about legacy stuff. I don't see how this can't happen to project in any language (PHP? Ruby? Perl? C#?)

That's news to me. Not that I disbelieve. Really interested. Can you point me to some use cases? Why do they not just use Windows?

Mostly on mobile I would guess. Xamarin has a showcase on their site: http://xamarin.com/apps .

Also every game made in Unity is using Mono, which is _a lot_ of games. The Playstation Vita and Wii U uses Mono as well for third party development.