My 1 Year old Minature Pinscher is micro-chippped, It was one of the first things I got done, along with getting all her Needles/Shots/Rabies Vacc. My dog of 15 years before (before her) was tattooed. I was going to have that done, but then I realized, technology has changed, and micro-chipping is easier for both the animal and the one trying to read it. I don't really have very strong views against it, I did it to be safe.
BTW- She's never experienced much discomfort from it, its not visable, and she hasn't had any problems with it. When I get another Dog, I'll likely have it done again.