I think the interface that brew provides is very simple and easy to use. The terminal output during installation is nice. Again, it's the extra little polishing of these OS X tools that I think really draws people to them and keep using them.
In regards to rbenv, the "core" part of rbenv(switching between versions of Ruby) works fine. It's the other tools like ruby-build and rbenv-install that didn't work for me. I don't mind manually compiling my rubies though.
Yup, I'm a huge mac fangirl, and then only thing I miss from the linux PC I have at work is a real package manager. apt-get, yum, good grief even gentoo scratches that itch for me better than brew or fink or macports
Also what problems did you have with rbenv on Linux? It seemed to work just fine the last time I used it ...