I bought a Nady PMC-3X for peanuts on eBay. It was so cheap so I didn't have very high expectations. But I'm very impressed. Audio quality is poor and it picks up a lot of wind noise, yes, but we can have an easy conversation up to 55mph no problem. Above that you kind of have to yell but that's no biggie. On the interstate is where it's limited, but it's still enough that she can yell "Can we stop at the next exit" or something like that.
There is also an upgraded headset for about $30~$50 that I'm going to get eventually to see if that's better on the highway. Of course, it does't use any sort of proprietary connections, so any headset that uses just a regular headphones-style headset jack will work.
It's battery powered. We got it last September. Replaced the batteries last week. Obviously, it's been winter, not a TON of riding, but I'm pleased with the battery performance. If all else fails, they also have a kit to plug it directly into your battery, into a cig lighter adapter, or into a trickle charger connector.
If you get their upgraded headset, it also comes with wires to plug it into two-way radios like a portable CB or FRS radio, and a handlebar mounted PTT switch. The intercom is always on, but when you hit the PTT switch you (or your passenger) will be transmitting over the radio, so it's equipped for bike to bike as well.
It also has an FM radio but that's useless. I can't get it to pick anything up. BUT, there is an aux-in jack that I use with my iPhone to listen to music, or get turn-by-turn from my GPS since I don't have a way to mount my phone so I can see it.
There it is on Amazon. There is a non-FM version, but it comes with a poorer headset and I believe it doesn't support the upgrade-ability to bike to bike. Just go ahead and get the version with the FM radio.
If you have any questions about it let me know. The box itself is small and light but big enough that my fiancee can control it with gloved hands. She is the 'keeper of the box', I don't mess with it. She keeps it in her jacket pocket, plugs our helmets it, turns it on, can adjust volume, etc., all with her gloves on. Then she just sticks it back in her pocket. It comes with a belt clip but I think it's easier to deal with in a jacket pocket.
Edit: I should also mention that the included headset only works with 3/4 helmets or better. It uses velcro to attach to the shell of the helmet inside the cheek pads. We wear full face, but I don't notice any decreased performance with the visor open and the chin bar vents wide open, i.e., I think it'll work just as well with an open face helmet. (If you wear half helmets you MIGHT be able to get it to work if it's one that has those big straps that cover the ears, probably won't be very comfortable though, it's hard plastic designed to go under the cheek pads)