I have this on a pillion/spare helmet, and use the 20S normally. Both are great units. The only downside? They are not water resistant and go absolutely berserk sometimes even in the lightest of showers. I personally expected more from the Sena units [but I still use them...]
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I've got the FM and non-FM versions of these on separate helmets, great little units, great battery life, sound quality's not bad either.
I've got one of these, at least the FM version (also available at xlmoto for £91), and it can connect to multiple devices. I've had it connected to my phone, satnav, and intercom. It can't play sound from all 3 at once (the satnav interrupts intercom/phone music). I paid about £130 for mine 4 years ago, but the prices very rarely come down on these so I'd say its a good deal if you don't already have a headset
Gets awkward doing bike-to-bike that way though.