Even with RubyMotion, you have to still learn the full iOS API. There is no way around that. You save a little bit of syntax, but Xcode is a pretty good IDE so that isn't the big deal.

The big deal is the community and culture of the open source that comes with Ruby folks. The number of open source RubyMotion applications is already perhaps higher than the objective C ones.

If I were to choose to build ONE iOS product today, I'd prefer to do it in the native objective C. But if I am a shop that plans to build quite a few of them, RubyMotion would be my choice.