I prefer the DVDs with the premixes. I do not have on-demand (due to horrible internet at my house and I travel a lot, so I buy the DVDs and the downloads. The downloads do NOT have the premixes.)
In the very beginning of my Cathe workouts, I did not see the value of the premixes, but now I seem to do premixes more than just the "regular" workout. Even if it is just a premix with the "regular" workout that adds an extended stretch or the abs. But I mainly use the premixes for the scrambled / mix-&-match of the premix options. Cathe does a really good job at creating premixes that are useful.
I also like the fact she will make a single premix from different DVDs/workouts. I know you can make your own mixes on her Blender, but I do not have the on-demand and I would rather the premixes already be made for me.