IMHO the most important part of the equation is the microphone. The headsets from eBay and the likes of Maplin are pretty rubbish, you'll only be heard up to about 40mph with increasing wind noise at anything above town speed making anything you say inaudible by others in your group. You won't know about it until they complain which probably won't be too long. These cheaper headsets are a false economy IMHO.
Decent Bluetooth headsets like the Senna and Interphone have noise cancelling built in though personally I don't think that these are as versatile as a decent wired intercom system. There are limitations to the number of users that can connect and, generally speaking, they all have to be using headsets from the same manufacturer as the long range Bluetooth is a propriety technology. Interphone have introduced the Tribe PMR radio to get around this but from my experience it's still far from ideal with volume issues and can only be used via Bluetooth as it has no mic or speaker of it's own.
Autocom, Starcom etc as recommended on here is the most comprehensive and adaptable solution and needn't cost the earth if purchased off eBay. These have decent noise cancelling mics and varying levels of features depending on model/price-point. Paired with a decent PMR radio and a Press To Talk switch you can get decent results for around £120-£150 for 2nd hand kit if you bide your time.
If you have a Bluetooth set-up and it does what you need then great, for me the only downside with my Autocom is unplugging from the bike which is now 2nd nature and not exactly a hardship. Upsides include bike powered system so no battery charging or flat battery issues and the intercom is configured the way I want with different inputs having priorities that work in a way that the only thing I need to do is use the PTT switch for transmiting bike to bike, everything else is automatic; sat nav, MP3, phone and incoming PMR. I have real world bike to bike range of 1-2 miles depending on terrain, maybe down to 0.5 mile in heavy urban environments.
As the OP is wanting bike to bike only I can recommend the
MaKay headset which plugs straight into the radio of your choice, you need to buy the appropriate adapter lead (CLAW). The open face version has a decent noise cancelling mic that works very well and can be used in most full face helmets. I bought one of these for friends to use and lent it one who subsequently gave me the cash on the spot as he wanted to keep it. He had previously used a cheaper non noise cancelling headset and the difference was stunning. I have yet to buy a replacement but won't hesitate to buy another from Wildtalk, both product and service are very good.
