I have no personal experience but Real Sex did a segment on them, this company that manufactures them actually takes couples out on a boat to umm, beta test the product, per say.
I can't even think about one of them ... I have this freaky imagination and I'm a real clutz (I generally don't use glasses, I use plastic tumblers). I just have this vision of the darn think breaking at a REALLY bad time
I know that's not logical but ... the image is there and that's it, ruined it for me.