What do you need it for??
My wife and I use the Chatterbox Tandem 50. It's really a no frills commo unit. The box is really simple and does it's job connecting the driver to pillion. it also has a plug in for mp3. The speakers are mediocre at best, but they are extremley thin and work in any helmet. I tried J&W speakers, and the 'Koss fix' and both were WAY to thick to fit in my helmet. BTW, I wear a scorpion EXO-400. The speaker volume works with or without earplugs up to about 80mph. Above that I gotta roll up the windscreen to hear over the wind noise. If you, like us, use it for direction confirmation or "I have an issue" minimal conversation it's a cheap alternative at under $150. Thats the box, 2 extention cords, 2 headsets. It was designed to clip to one helmet, but she likes it better clipped to a belt, or in a pocket, hence the 2nd extension cord.
Chatterbox's major drawback is that the battery is internal and rechargable only like a cell phone. Can't just toss in a new pair of AA's if you leave it on by accident. Ours lasts about 12 hours before the sound quality tapers down and takes about 4-6 hours to recharge with the supplied AC wall charger. There is also a wire to power the box from the bike battery for about 15$
One thing I like about the chatterbox is they make a cord that is the 6-pin connector on one end and a stereo jack on the other, giving mp3 capability without having to use the actual chatterbox 'box'. connects the iPod to my headset directly. nice for those long solo slab runs since I have the speakers in the lid anyway.