The only gems that do not work are those which are not pure ruby. For the popular gems I think there have been rewrites. I haven't looked into JRuby but it has changed quite a bit in the last year from what I read
I don't think that's true. There are plenty of threadsafe gems with C extensions, and plenty of non-threadsafe pure ruby gems that mutate class/class-instance variable willy-nilly.
