Personlly I've never tried one. The treasurer and I have been thinking about getting bike to bike comms, but there's other bike related bits we want first.
The wind noise problem varies with the brand. A couple of guys I know (HD riders) have 2 way and reckon it's pretty good, but I don't think they have voice activation.
Hand signals are good, except when you're doing the tourist thing and you point to something interesting and the other person misses it.
As for Rider to Pillion comms, forget it.....the reason we take the kids with us on the bikes is so we don't have to listen to them whinging.