Just got 6 berghias from reeftown.com. They say not to expect much for a few months until they've reproduced through several generations but I've noticed that the rock where I introduced them is already aiptasia-free. One thing I'm hoping with the berghias is that they get into my overflow and my sump. I've had peppermint shrimp and a Copperband in the past and they both did a great job in the tank but obviously weren't able to get anywhere else.
I don't know why they say this. They've always done the job for me within a month, and usually within a few days!