Got one last week via Ebay...
From talking to friends, they say the sound quality is very good even at 70mph.
The describe it as sounding as good as a Car kit at medium to high speeds and maybe even better than the normal handset at low speeds!
Earpiece is very slim so fits in easily on the helmet.
I don't think the boom piece looks very hardwearing.
Not sure about the waterproofiness of it. they describe it as good for rain or snow, but not sure how it would cope with the biblical rain we got a few weeks back on the Brownie points run.
Only real problem I've had with it is the voice recognition of it.
The phone doesn't seem the recognise my voice, and even when I try to record the voice wearing the helmet, I get a "go somewhere quiter" message from the Sony Ericsson K800i.....