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.