It works best in Thin (EventMachine) or Rainbows! (Fibers).

If you are doing I/O you should use NeverBlock (http://www.espace.com.eg/neverblock/) they've even implemented a MySQL driver, the best part is they're 'drop-in' so you don't have to learn a new API, they just hide the async calls from you.

Fibers I think are closer to Erlang style than the event loop, and much more straight forward to program.