I've been using the 30 DS for about 10 months, actually, but I use it as part of an overall rotation. I include a couple of her other DVDs (No More Trouble Zones, Banish Fat Boost Metabolism, and Yoga Meltdown) as well as some old Turbo Jam DVDs I have. I've also used a couple of the Firm DVDs. All of the DVDs I have I've gotten at Target or Wal-Mart.
To be honest, "rotation" makes it sound like I have a planned out schedule. I definitely don't.
On any given day I see what video I haven't done in a while, and do that one. Hey, it works for me! And it has the added benefit of keeping me from getting too bored with any one DVD.
Another tip a friend shared with me: Put the video on mute, and listen to some good music while you're working out. I love Jillian, but there's only so many times I can listen to her say the same thing - ditto for any instructor out there.