Buffy - I remember I caught the first few episodes and thought it was alright, but then stopped watching it. Years later I caught an episode late at night on TV, which turned out to be the final episode. I went and got all the boxsets, watched it all, and started kicking myself for missing the awesomeness of it.
Scrubs - I saw it on sale for $25 Season 1. Brought it, watched it all, and immediately went and got Season 2. By the time I finished that, it was 2 weeks before Season 3 was released, so I had two whole weeks to wait to see what would happen to Cox. I was hooked and haven't stopped watching.
HIMYM, TaaHM, NCIS, Friends...the list goes on and on. I might catch an episode here or there on TV, but I mainly decide if I love a TV show based on when its DVD is released, and how much I love the DVD, and thats usually years afterwards.