I was running a specific rspec over and over during a tight development cycle. The way our system was set up it was taking 30+ sec to run a single spec file, which made running them onerous.
We had Spork running at one point, but it wasn't working for me, with a "undefined method 'flush' for nil:NilClass" error (and a couple of others depending on the incantation):
After upgrading Spork (we had been running an older version), setting the JRUBY_OPTS environment variable, and restarting Spork, the spec time dropped to ~8 sec. Still not particularly fast, but faster-enough to make it worth it.
Not currently running Guard. Not sure if the upgrade or opts made it start working.
This post is a work-in-progress.