We've had this for a few days now and finally seem to have mastered getting the rider to rider communications to work properly. It has to be said that once set up it's very good with some limitations but the set up is picky.
The bigest problem has been to get the rider to rider to play nicely with the Zumo GPS, I'm not sure if that's completely solved yet. I tried yesterday without listening to music and that was a big help.
Once installed in the helmet (and that's a fight) it fits very well and the only issue is to make sure that you route the microphone stalk away from your cheek when you close the visor. There's not a lot of room in the helmet (something we like) so introducing extra kit (like the mike) can make it a bit claustrophobic at first.
Distance, as you might expect, is an issue as is line of sight but we were able to keep in contact on the Takaka Hill where visibility between us was not always possible. Get too far away and the voice becomes unreadable long before the connection drops.
You start a connection by making a loud sound into the mike and it responds with a 'bong' to let you know it's on. Another 'bong' let's you know it's off after a period of silence. So there's only a small chance of muttering about the other rider being overheard! Normal volume speech won't set it off.
The real score is in gas stations and other stops where you can talk without shouting at each other although you get to overhear one side of the conversation with, say, the cashier. Somewhat disconcerting.
Battery life seems good for a days ride too.