The major concern I have with something like RubyMotion is that Objective-C - the language it stands in for - is not a difficult language. The Cocoa/CocoaTouch API and its verbosity is the real problem, and I think that trying to map the extensive documentation written in Objective-C to a Ruby equivalent runs the risk of adding too much mental overhead. That it requires a paid-for license means that Ruby specific documentation will be comparatively thin on the ground, and the same applies to the resources found on StackOverflow.

I'm a Ruby developer, and I find Obj-C a nice language to work with (though I do prefer the s-expression style in favour of the more recent dot notation). I still haven't managed to justify using Ruby in place of Objective-C.

The only major benefit I see is the REPL that ties into the simulator, and losing the dependency on X-Code, but again, I feel that the verbosity of the APIs is the fault for that.