MRI is a steaming pile. (Note: You're not allowed to disagree with this unless you're a C programmer that's spent some quality time with MRI's code.) It seriously shook my faith in Ruby the first time I had to dive into MRI's code: "Could someone who waxes poetic about elegance and fun in coding really produce something this fugly? Does Matz have any idea what he's talking about?" Rubinius has a sane design and is a pretty clean base to build a Ruby implementation with some staying power on. I basically see Rubinius as the rewrite that MRI was going to have to have at some point.

That said, at present, I still use MRI. I try Rubinius every couple months to see how it's progressed performance-wise for the cases that I care about, and while it's not there yet, this seems to be mostly a matter of time.