Does it let you write for all those platforms with the same language calling the same API? I'm not familiar with C#/Mono besides hearing news about them periodically. If I have to rewrite my code to use a different API for each, it might as well be a different language, since it's the rewriting part (or lack thereof) that's important to me. But if I write once and run everywhere, C#/Mono might be kind of compelling.
It depends by what you mean by API - it's not a "Write Once, Run Anywhere" UI framework, because you wouldn't want that anyways: an Android app should work differently than an iOS app, full stop.
However, everything else does work 95% the same - how you access files / databases, your business logic, all of the non-UI libraries are both consistent, and work really well.
