I love TV series on DVD. I get to watch stuff I was never able to as a kid (Greatest American Hero) or stuff that I didn't get the channel for (Buffy/Angel)
I did buy 24 Season 1, but I sold it quickly thereafter. Why? The whole premise of the show is shock/suprise. Since I'd watched it once already, I couldn't watch it again with the same degree of fascination. I rented seasons 2 & 3.
Angel/Buffy, OTOH, I have all seasons except Angel season 5, and I'm debating not getting that. The first half of the season was excellent, but the second half sucked. Good reason for that - they cancelled it halfway thru, and they had to scramble to re-write everything.
FireFly, Joss Whedon's other series was cancelled after only ten shows aired, but they shot fourteen. Darn shame, it was a *fine* show. Good thing they're shooting a movie based on it.