There good, cause they eat left over food. there called bristle worms, just make sure they don't grown to big cuase i think they've been reported to eat small fish, but only when there all grown up. they do have spikes on them so dont touch them.
I wouldn't exactly say rare, but I would say hard to come by in the hobby. The little wavy ones are probably spagetti worms, also harmless and beneficial to your tank.