I would like to try it out but I'm not on Windows obviously, and I won't work to become a second class citizen ("What do you use? Mono? That's not supported").
Also, most of tooling seems to be proprietary, Windows-only too.
I'd suggest you give Mono and MonoDevelop (http://monodevelop.com/) a try! It's really good and at least most of the web stuff that I've developed have just worked out of the box on Linux (Apache + Mono).
There are real companies out there making real money off Mono. So it's not a toy reserved for weekend tinkering. Enough companies trust Mono to use it in production.
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#?)
Also, most of tooling seems to be proprietary, Windows-only too.
So I'd stick to JVM.