My wife and I use the Blue Ant - Interphone system for Helmet to Helmet / Bike to Bike. It uses bluetooth to connect between the two or from the user to their mobile phone (phone needs to be bluetooth capable). The unit fits well on the outside of the helmet with a single sided speaker and boom mike on the inside (suits open and closed face helmets). Fully waterproof (although I have not tested it).
We found that it is easy to install on helmets with removable cheek pads but difficult on my old helmet that had fixed liners.
The volume and mike work well even at highway speeds. Has voice dial (if supported by your phone) and is a full duplex system so you can have normal conversations (not like the old CB radio - one at a time system).
The unit is rechargeable and we get around 4 to 5 hours of constant use before a 2 hour recharge. No option for running it off the bikes power socket. System comes with a wall charger and a in-car charger unit.
The only gripe I would have is it is a little tricky getting it to connect (helmet to helmet) each time. It does remember the unit and phone it is paired to but when you start you need to go through a single button multi push, listen to beeps etc. etc. etc. to get to the point where they connect. They have a flashing led on the unit which is a little stupid because you have the helmet on and you can't see it.
We (my wife and I) find ourselves standing beside our bikes, helmets on, pushing buttons, looking at each others flashing LED's and repeatedly saying - "can you hear me now". Best done off to one side where the general public can't see you.
Just when you think you have the process nailed, you forget it all again before the next ride.
It is fantastic though. Our friends comment about us when we all go riding. Apparently my wife and I do little nodding and head shaking actions while we ride, as if we were talking face to face. Looks strange but we love the freedom and the fact that we can better share the ride experience as we go. Would be nice if it provided more than two connections. Party calls would be interesting.
Anyway - we paid in Australian dollars about $210.00 each. Not cheap but they work and we love them.
http://www.blueant.com.au/interphone-mo ... et-kit.htm
No, I don't work for the company and don't get a kick-back. It's just a cool thing.
P.S. No connection to MP3 players (iPod etc.) unless you have some additional bluetooth unit for the MP3 player. Even if you do, the speaker is for voice not music.
PPS. I have used it with ear plugs on and it works fine, so possibly headphones (in ear) for the MP3 player may be OK but I haven't tried it.