Oddly, I didn't run away from Objective-C and Xcode screaming. But I have run screaming away from Ruby on Rails a number of times. I do acknowledge that this is likely a personal shortcoming though.
But Cocoa is the thing you have to learn no matter what. To be honest, all things considered I would rather put up with Objective-C than lose Interface Builder and the niceties of XCode.
And I say that even though I have used and love PyQt. Cocoa is so deeply married with Objective-C and XCode, I can't really see the point of using RubyMotion.
Disclaimer: I've not built anything extensive with Rubymotion/IB, but I have built a proof of concept. I don't know if it's really feasible on a large project.
Also, Obj-C and Ruby share many common concepts, using Cocoa from RubyMotion usually requires just a bit of syntax swapping, "vanilla" RubyMotion is not much of an abstraction.