Cat, I know but also applies to dogs.
Long story short, a good friend had her cat go missing last November. She tried absolutely everything to find him, posters, flyers, the lot. He was micro-chipped.
This morning (4 months later), she had a phone call from a vet 15 miles away on the other side of the city to where her cat went missing. He had been taken in to be neutered as a stray, the vet had discovered he'd already been done, scanned him, found he was chipped, checked with the microchip company and the result is he's heading home to my friend this evening. She is picking him up after work and is utterly over the moon with delight that he has been found safe and well. :)
Absolute PROOF that microchips can and do work to get lost animals home.