I used to have one on my 1150GSA for when I was providing rider training. I powered it via the bike and had a PTT switch on the left-hand controls:
It was run through an Autocom that would mute my iPod music whenever I transmitted or was receiving. I fitted an Autocom boom mic to my helmet but preferred my Ultimate Ear monitors to helmet speakers.
The unit had the ability to double the signal strength (twice the UK permitted power) as long as you mounted the optional, short, "liquorice stick" antenna.
Was it any good? Well I only really required one-way (from me) comms during instruction and it seemed fine for that. I did use it on occasion to chat to a mate who's bike was similarly equipped, when doing some touring.
In the latter case it was more of a hindrance than a help. Firstly, if I wanted to chat to a travelling companion on the road I'd travel by car. Second; the signal is never as clear as the sort of radio comms I was used to in the old days at work. I'd get half a message or just a carrier-wave and have to stop to be certain that it wasn't an important message, more often than not it would be something like, "did you see the tits on that?" and the journey was unnecessarily interrupted. Sometimes what you receive is break-in from other sources, you stop to ask what was said and find that neither of you has said anything!
The range is too short to be of real use if you do become separated, you're better off relying on a mobile phone.
VOX operation is available on many but then you get people who don't realise that the first part of what they transmit is lost because the voice operation needs a pause to wake it up. Or worse, every time your buddy clears his throat it opens the mic and you think that you've missed something he said.
Nothing is worse than having your mate accidentally go onto permanent send and having to listen to him singing along with his music (even if he is a choir member and quite accomplished).
It can be done, it isn't cheap though.