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.
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.
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.