Been doing some research for the "ultimate" system to manage all the communications and media on my bike. Minimum requirements:
Stereo in helmet headpohones
Blue tooth connection to GPS (TomTom or Garmin Nuvi) - Navigating unknown towns in the US is so much easier with the audio prompts from the GPS
Blue tooth connection to cell phone
Hard wire to iPOD
The short list consists of the Scala Rider Q2, and adding the an extra base unit that will accept an MP3 cable. This gives me two base units for my two helmets. I've used the Scala Rider in the past and was amazed how clear conversations were via cell phone. Person on the other end hardly knew I was riding.
But there are other systems out there, namely Strarcom (http://www.starcom1.com/usa_price.htm) and others I am sure.
Bike to bike or bike to passenger is not that important. So if there are other members here that have used or are currently researching similar devices, I'd love to hear what you have learned.