I have both of mine chipped.

It was $15 for the chip and to register was I think $18, but for two or more it was $30. My vet told me that any pound/shelter, etc that has a scanner will be able to read the chip.
I can not feel the chip in either dog, they have never been bother by it at all.
The thing they use to put it in their skin looks scary... but it's over in a second. Neither of mine even flinched!!

What they put in is a rice size microchip with a number on it. When someone finds a dog and it's scanned, it decodes the number, then they call into the company with the number and the company give the owners info. You have to give an emergency contact number (incase your pet is found and they can't reach you) And if you ever have to change your info, ie... if you move, or change your phone number. They charge a fee for any changes I think it was $8. I know the company mine are chipped with is called AVID.