I didn't bother using MacRuby as a stepping stone to Objective-C. Instead I learned C first. I had no experience with compiled languages so I figured it would be my best bet and I definitely think it helped a lot.

It's a little hard to get past Objective-C's syntax at first but after it all begins to click it's not so bad. I think learning the Cocoa API would prove the more difficult step in your learning path.