I have two USB rigs - one with Gekkos and one with MoonLanders... and, yes, they are fun and efficient. The fun is seeing what you can do with the clocking and voltages to get the best results. They are a challenge to get running, but once you have 'em going, they usually are stable and you don't much notice the bump in the electric bill.
BTW it's a 49 port hub, but there are only about 22 USB sticks currently running in it - I sold a few off so I could clock the remaining ones a bit higher. I have to reboot the USB rigs about once a week as opposed to the bigger rigs I have which require more intimate care and handling.