My vet does not recommend Frontline.  I used it two cats.  It didn't do anything to help Cat #1.  Cat #2 had a bad reaction to it within a few seconds (drooling, gagging).  I went back to Revolution (Stronghold) and all has been well since.  I also keep a package of Capstar around, just in case I see a flea.  It starts killing within 20 minutes, and gives 24 hours of protection.  
I've got a special-needs cat who prefers to potty outside, and needs outdoor playtime, so flea control is a must for me.