I liked them when it came to buying an actual hard copy of an album.
You had the music on one side, and some of the DVD features on the other side. (Which possibly had music videos, short features, performances, etc.)
Instead of downloading, this actually gave me more of a reason to spend $15-20 on a CD.
I have like 3 or 4 of these. I think they are cool, even though I don't really use the DVD side of it. I guess it's for feature reference. I just don't care for that part. I was thinking about getting ITZ DD. But nah....
I think they stopped making them because people were having problems playing them in certain players. I wanted to play Rob Thomas' album in my car and it didn't work. I think the last dualdisc I saw released was Weird Al's "Straight Outta Lynnwood".