But the real payoff is threefold:
1) You can use and write very ruby-ish wrappers for things. A great example is teacup, which lets you essentially write stylesheets for your UIViews (and I guess now NSViews).
2) Working with collections like arrays and hashes is MUCH more convenient. You can spend a good part of your time doing just that.
3) Command-line toolchain. It's not for everyone, of course, but for those who work better like that, it's a revolution.
Right now it's pretty wild west. There's no obvious winner in terms of libraries for the data layer, for example. So taking on an RM project right now either means rolling up your sleeves and hacking on the infrastructure, or just living with raw Cocoa, without benefit of autocomplete. Give it another 6-12 months and I think you'll see a very strong contender.