Nice! The big caveat here is that they're cooperatively-scheduled, and they've left the I/O stuff up to libraries. In other words, they look exactly like Python's greenlets, which are very useful when you add the appropriate libraries on top.

Edit: the other big issue here is that blocking operations (e.g. anything in libmysql) are not automatically deferred to a thread pool -- that has to be done manually. Ruby's fibers and Python's greenlets are magical, but not magical enough that you don't occasionally get tackled by fantasy creatures materializing from the aether. If that makes sense.