I use Advantage on my cat and K9 Advantix on my dog. I've kind of fallen behind schedule on giving them their next dose because we don't really seem to have flew problems. The only reason I started them on it was when Addison had tapeworm that comes from fleas. He very well may have had it since he was in the shelter though.
I've always used Advantage on my cats - it's what the vet gives me when they need it, and after a HORRIBLE* experience while visiting a flea-filled house in Maine, it works for me.
*I first used a "natural" flea collar on them - a cotton collar that you applied a natural flea repellant oil to, which did not work at all. I ignorantly applied some of the oil to my cat's feet, which he immediately started licking off, then FREAKED OUT because it tasted so bad. The whole thing ended with me in tears, thinking I had poisoned my cat, on the way to the local vet. They gave me Advantage there, too. Needless to say, that was the cats last visit to Maine.
I used Advantage a couple of times but then switched to Revolution. My kitty's only 6 months old, but Revolution seems to have worked really well. My vet did say he likes Advantage better tho :? But Revolution also protects him from heartworm, ear mites and other parasites, so I figured I'd go with that.
They both last for a month, then you have to re-apply.