I have been looking into these too, as we haven't had very much success with other intercoms.
I can't find the website I was looking at the other day with all the info.
you buy the helmets with no accessories.
the first thing you need is are headsets, which do have the advantage of being designed for the helmet and the helmet being designed to take it.
If you are going to just use wired comms you buy an attachment for the rider and the pillion plugs into it with their own headset.
If you want to use bluetooth you need a bluetooth box for each helmet and they should work up to about 30 feet apart or you can use your mobile phones.
It looks as though you can also use the headset with a normal radio unit and if you can get vox to work satisfactory you don't need a ptt button.
You can also plug in your mp3 player or satnav.
I'm not sure but I got the impression that although the bluetooth is only mono, anything through a wired mp3 player would be in stereo.
So I was wondering if 30feet was enough for most of the time you want to talk with using the phone as a back up if you get too far apart or split up.
I'd like to hear from someone who has one and uses it successfully.
If you think you are too small to make a difference you have never spent the night with a mosquito.