We got this a few days ago, and used on yesterday's ride: http://www.amazon.com/GSI-Waterproof...ef=pd_sbs_hg_1 .
It uses a pair of AAA batteries, and comes with a couple of ear buds with microphone booms.
* the box is small, we clipped it to the back of my shirt collar - at the label. Good location so that Diane could plug in/out when she got on/off, and I did not even feel it.
* we could hear each other, even along the road with heavy traffic.
The not so good:
* the ear bud mic combo is intended to be velcro'd inside an full coverage motorcyle helmet - doesn't work so well on a bike.
* the hook up (single three conductor 3.5mm plug) is different from our computer (Skype) head sets.
We have ordered a couple of different kinds of headsets to try.
You can also attach a CD/MP3/whatever to it for both people to listen to. We probably will never use that feature.
Bottom line: great price, works well, but plan on spending some more money on different headsets. Even then, it will be a sub-$40- communications solution - a great bargin in my book.