I had Leo microchipped at 6 months when he was at the vets for neutering. I think this is a good time as they can do it without pain. Leo's chip is the Home Again and they are proactive about finding lost pets. I would recommend it for any pet to help get them returned if they are ever lost.
Gandalf and Gwenny are microchipped as well as our mini mutt, Charlie. Given that I just saw 3 corgis on Petfinder.com as strays with no chips, I just think it makes sense. Especially since it really isn't very expensive most places.
Wrigley was micro chipped at the shelter we got him from for $20. I think it's worth it. I worry about him slipping off his collar and getting away from me or somehow getting out and this way, I know he can always come back to me.
Roslyn has a microchip. The rescue we adopted her from puts them in all their dogs and pays for the first year with Home Again. I hadn't actually read about any health concerns with them, so i will have to check into that, but i will tell you it gives me more confidence when we are on walks or at parks. Roslyn has a leather break away collar because i am concerned with the choking risk, but i was always concerned about getting her back if she did run away. It is a small price to pay for peace of mind.