Rails compatibility is apparently on the long-term roadmap, but I believe it is a long way off yet.
Using 0.4, I was unable to get any of the mainstream testing libraries (especially RSpec, and including Test::Unit) to work properly, and I struggled with simpler alternatives such as Bacon. Not sure how 0.5 will fare with these yet.
It's really cool that 0.5 uses LLVM. Does anyone know how this it stacks up against the other Ruby implementations?