It should have ended with the 3rd season. The 4th season was directionless, the 5th season was ok, the 6th season again went nowhere (not to mention the stupid magic-as-drug subplot and Tara's death), and that's when I stopped watching.
Season 5, where Dawn appeared (confusing me to no end - I was like, hold on, it's impossible that i've missed an episode, but this is making no sense at all...) shook things up and was great.