I have lots (
too many DVDs really

).
Mostly I collect the series that are no longer on air. To my mind, it makes the most sense. You can have the complete run of something that is no longer available, and who knows if it'll ever be available in rerun. This way, you can watch any of it, any time you want.
Babylon 5 was the first one I went after. Then Farscape. I also got Red Dwarf based on a few eps of the first season when my friend brought his DVDs. Picked up Buffy and Angel after they ended. Dilbert was at least sold as a complete series box set. I rented Dead Like Me and decided they'd be worth picking up for being such a black comedy. You don't find too many of those outside the UK. Speaking of which, I have Mr. Bean and intend to get Blackadder.
Then there are series which are still active but not rerun. I got Battlestar Galactia (new one) because of that. Same for Alias (at the time). Doctor Who 2005 season and will have 2006 (offical) when it comes out. I got the better seasons of Sealab 2021 because it's so twisted

Same for Drawn Together and Tripping the Rift.
I try not to have too many ongoing series because they're so hard to shelve what with new sets coming out every year. I picked up Family Guy because it was discontinued at the time. I ended up picking up the South Park series because I found a few seasons for cheap and am a completist. Same goes for CSI: Las Vegas (though that's not bad as I missed seasons 5 and 6 and they're not in mass syndication yet). One of my fav series is Law & Order and I have the first 4 of that (don't have season 14). And I just recently caved and got the first 7 seasons of Stargate: SG-1. (At least I'll only need 3 more there if rumours are true.) Oh, and Robot Chicken (so random!)
Probably the final and best argument for TV series on DVD is that they have the highest length to cost ratio. A full season of a 1 hour (45 min average episode time) show (22 seasons) will thus give you about 16.5 hours of solid viewing for between $40-100, or a cost of $3-5 per hour. Compare to your average movie (90-120 min) for $10-30 and it's easier to see which is the most effective of the two.