I bought a set of generic BlueTooth ("BT") intercoms off ebay for use during WDW and they were ... fantastic!
For a pair of headsets it cost £52 delivered. I went for a specific type/look and a better range (800m instead of 500m) set, as this was the sweet spot in price vs spec.
Although the set I have come with fixed mic booms, they fit easily into both my Arai Astro-R and Arai Tour-X. The fitting kit includes all sorts of options such as double-sided adhesive pads and a clamp kit for the side for the main unit. The speakers are velcro backed and fit fine on the helmet lining. I spent some timing running the cables behind the padding/liner to keep things tidy.
The BT unit has a power button, a "jog wheel" and a central button in the jog wheel. The power button turns it on and also connects to any other headsets that are paired. There is a one-time pairing process, but after this it "remembers" other headsets and "calls" them when you press this button once.
The jog wheel allows you to turn the volume up and down by rotating and holding the wheel.
If you pair the BT to your phone, you can also take calls and listen to music. When listening to music it will skip to the next/previous track by turning and releasing the wheel. The central button on the jog wheel starts and stops music playback. If you get an incoming call you can answer/hang up with the jog wheel button.
It's really easy to use and waterproof too. I can listen to music at any speed (tested over 100mph!) and I wear ear plugs as well. My iPhone has a separate volume setting when paired via BlueTooth, and I just had to turn that up to almost maximum, then use the BT volume control to turn it up/down.
For bike-to-bike they are ideal and you can easily hear the other person. Things would get a little garbled over long distances, and Andy had trouble hearing me over about 60mph but that may have just been either my microphone or his speaker settings as I could hear him fine! You simply press the button and it connects you to the other headset. Chat away - full duplex (as in you can both talk at the same time) - and when you're done press the button to disconnect.
To recharge you can either remove the unit from the helmet or leave it in place. You use the same socket that the mic/speakers plug into to charge it, so I just pull the mic/speakers lead and put the charging lead in overnight. I can get about 4-5 hours of music between charges.
In 26 years of riding I'd never felt the need for a headset, nor music on the go, but having ridden with this round Austria and Italy for WDW, I've kept it in the helmet and used it every day to listen to some background music on the daily commute.
Well worth the money!
I've attached a pic of my set on my Arai Astro-R. They are very similar to these ones on ebay:
http://r.ebay.com/WZnXUn