I'm the biggest Seinfeld fan in the world but the show definitely started to go off the tracks after the original writers - Larry David, Larry Charles, Peter Mehlman, etc. had less of a hand in things. The style of humor totally changed, as did the way the actors portrayed their characters. They became over the top impressions of themselves. Jerry sort of just sleep walked through it all. Elaine went from at least likeable to sort of a bitch. George went from whining and neurotic to just yelling all the time. And Kramer went from a certain subtle cluelessness to just being a spaz. The later seasons had their moments here and there but, generally speaking, stick with the first 5 seasons.

And by the way, you'll notice the exact same thing happened to The Simpsons.