I tried out the Scala top of the range bluetooth intercom with a friend and found bluetooth to be, well, shit. We tested them relatively carefully and they managed about 600 yards clear line of site. Through a building/trees/fences etc round a corner they managed about 50 yards (both of us outside, house in the middle between us). For all the convenience (over radios) of having the units self-contained on the helments, the range was dismal. So we went back to (legal) analogue radios. Slightly less convenient and more expensive than a cheap bluetooth unit, but very reliable, no digital artefacts (dalek farts!) and a range (depending on terrain etc) of 1/2 mile - 2 miles or so. Less of a range in built up areas if there are buildings between us, but not as hopeless as bluetooth (which, incidentally, is a really crap technology for lots of other reasons). We even managed a range of about 16 miles one evening, with me stationary on a high hill.