I had JC microchipped - it cost me around $14 - $15 at the going exchange rate. Thus far we haven't had to use it, but you never know. There was a story in Cat Fancy recently about how a guy was reunited with his cat that had disappeared ten years earlier - possible only because the cat was microchipped.
In this country, laboratories can't buy and use dogs and cats that are tattooed or chipped, so I had both done to Jamie, the former when he was neutered. I also have him entered in three different registries (two German, one European). The German registries didn't charge anything, and registration in the European one was included in the implantation fee. The rates seem to vary in the U.S., though. My mom paid a little less than I did for her dogs' and cat's transponders, while some people here on the forums have paid much more, or gotten much higher quotes from vets. It probably pays to call around and ask.
Implanting the microchip is a matter of a few seconds, and doesn't seem to hurt as much as some inoculations.