It's not as good as it is on Windows, simply because of Visual Studio (let alone Visual Studio plus ReSharper). I must admit, it's enough of a productivity improver that even on my Mac I prefer to do most my hacking in VS running under VMWare. But that's not really anything against MonoDevelop, it's just that lately Visual Studio's gotten so good that it's really hard to resist using it if you already own a copy.

Compared to most other open source platforms, OTOH, I don't think it feels like one's been relegated to a language underclass at all. Of course you don't get the benefit of working in the operating system's official first-class language like you do on Windows, but in fairness that's a privilege that you never get to enjoy when working in most languages - including Java, Python, Ruby, and so forth.

As far as why it's less popular outside of Windows, I'm inclined to think that's mostly down to social effects. Java's got the Linux enterprise development space so wrapped up right now that I suspect trying to pivot to Mono would be a bit like trying to push a glacier. Businesses aren't in the habit of rewriting millions of lines of code on a whim.