All of our cats except for our feral have been microchipped (I plan to get her done next time she's at the vet). My 3 older cats' chips were implanted back in '95 when I first heard about microchipping from the vet. DH's cat was done probably 8 or 9? years ago, and my dog was done when I adopted her 4 years ago. My 3 older ones are now 19 and 13, and DH's cat is 11, and they're all fine - no problems with the microchips.
We use the AVID brand, which is what the local pound uses. Unfortunately there are several different chip/scanner manufacturers (AVID, HomeAgain, 24PetWatch, Banfield, and possibly more), and there may be some compatibility issues between brands & scanners. All scanners should
be able to tell if a chip is present, even if the scanner can't read the chip because it's a different brand - the person scanning would then know to try a different brand of scanner, or a 'universal' reader. I would recommend calling shelters in your area and finding out which brand of scanner or chip they use, and going with that brand so you won't have to worry too much about compatibility issues.
It's very important if you get your cat microchipped to register your contact information directly with the chip manufacturer's registry service. Unless you submit the info directly to the chip registry, they will only have the info to your vet clinic, and the pet's finder may have difficulty reaching you. It's also generally a good idea IMO to keep a visible ID on pets such as a tag & safety release collar, in case a pet is found by a person on the street, so they can hopefully read the info & call you immediately without having to go through taking the animal to be scanned. Also, some people still aren't aware that microchips for pet ID exist - a visible ID can be helpful in those cases.
I'm not sure what to think about the tumor stories, honestly. But after seeing the lost-pet heartbreaks that accompanied hurricane Katrina (a lot of animals came to this city for rehoming), personally I think a certain amount of risk may be acceptable, unless a clearer link to tumors is shown.