It's More Than Just A Music Store. iTunes' Smart Playlists are an incredibly powerful tool that I use constantly, and it seems I'm not alone. In fact, it doesn't sound like I'm nearly the most advanced user of playlists!
I've got around 140GB if tunes organized in iTunes right, and I've got both positive (Highest Rated, Most Played) and negative (No Year, No Track #) playlists set up. The negative lists are mainly there to help me fix the tags on really old albums that I ripped back in the days when Audio Catalyst was the state of the art. I use the Disc # field in a different way than most people do, I suspect.
The obvious intended use of "Disc #" is for multi-disc albums. For that, I put ", Disc 1" at the end of the album title, so that each disc is considered it's own album. Instead, I use Disc # to record to which CD-R media I burned that particular album. For me, the normal lifecycle of a CD purchase is to tag the album, rip the album, start listening to the album in the background, drag the album to one of my to-be-burned genre dumb playlists, and eventually burn that dumb playlist to MP3-CD when it gets reasonably full. The process of burning starts with making a note of the most recently used Disc #, incrementing by one, and setting that Disc # for each of the albums to be burned. I then burn the album using Toast, since iTunes unfortunately throws all of the tracks burned to an MP3-CD into the root of the disk, which my MP3-CD players don't like so much.
Needless to say, I've got a smart playlist which contains all albums with no Disc # on file.
(This article also appears at W6 Daily.)