Kim and I bought a pair of these for our trip to Nelson and Osooyoos this summer. With Kim being a new rider we felt it prudent to be able to keep in touch on the 8-hour riding days without resorting to ambiguous hand signals.
Well, they worked great. Ever better than we imagined. They took away so much of the stress of paired riding because we were in constant contact. No missed turns, no ambiguity about filling up or the reverse.
We did a trial run using VOX (voice operated transmission) only, but it was a little frustrating. With the wind noise on a motorcycle we had to have the sensitivity turned down, which meant that we were losing the first word of what we said.
Fortunately, the units come with a PTT (push-to-talk) extension cord and handlebar mounted button. This is the thing to use. We each mounted the button on the outside of the left handgrip and it's super easy to reach a pinky over to transmit.
The X1s also have stereo input so we've been listening to our iPods on longer trips. Note the music doesn't transmit, it's just played locally.
Some might worry that putting a radio on the side of your helmet would be bothersome, but we haven't noticed any problem with that. You hardly realize it's there.
Batteries are NiMH or something not NiCd, so you don't have to worry about developing memory when charging. They seem to last about 8-9 hours of continuous use, with an iPod playing the entire time. They'll probably last longer if you don't play music.
Downsides? Rechargable batteries are nice, but it would be nice if they also accepted AAs for if you ran out of juice on the road. Other than that, which is pretty unreasonable in the first place, nothing comes to mind.
Recommended for people who travel in groups of riders. Note if you just need rider to passenger communication there are less expensive options. Chatterbox doesn't have a site that allows deep linking, but here's the URL:
http://www.chatterboxusa.com/ Look under "Rider to Rider."