I added about 20 to my 100g about 5 months ago and they quickly finished off my aptasias. I didn't see much of them for the first 2 months or so now they come out all the time. When I recently added 2 gobies (about 3" long) to the tank I was actually a little worried because they pestered the hell out of the gobies. The would actually corner the gobies and try to pick at them.
Anyway they've gotten better but now I have 2 tiny barnacle gobies (1" big) in QT
and I'm a little bit worried about adding them to the tank as pretty much all of my shrimp are bigger than them.
They've actually become as annoying as the cleaner shrimps. I have to wait around (and fend off with a stick!) for my anemones and corals to "swallow" their food otherwise the shrimp get it.
I've heard that a lot of people haven't had much luck with these shrimp and aptasia but I don't have a single one left (that I can see anyway).