I have been able to spend a little time with this product recently and it still impresses me.
The one thing, though, that 'steepens' the learning curve is a need to fully understand the iOS framework and GUI toolkit. Much of the syntax has been molded to Ruby (although, to their credit, you can easily mix-and-match between Objective-C and Ruby) but the fundamental structure - of course - is still iOS.
The samples provided on Github offer a great variety of techniques and ideas from which to learn and to build.
I wonder if anyone else has had some contact with RubyMotion or has any ideas/impressions to offer.