I own the DualDiscs with 5.1 mixes of the first six albums (Debut, Post, Homogenic, Selmasongs, Vespertine, Medulla--basically, the ones included in the "Surrounded" box set, though I bought them separately), and the DVD-A of "Volta." The music is great but I find the quality of the surround mixes to be wildly varying. Not impressed with the mix of "Post" (but still better than stereo!
), very satisfied with "Vespertine," the rest somewhere in between.
As for whether the sound quality is up to snuff, obviously they're not going to be as good as SACD. I don't know if there's any difference between any of the DualDiscs and the DVD-As.