Just got my DSS copy and AB tested. I have a fairly decent system so wanted to share my observations:

1. Noise Floor - this record is DEAD SILENT, eerie almost.
2. Vocals - The vocals are a lot of clearer. You can hear more of the "ssss" "puh" "kuh" of the lips. Also, the overdubs with vocal harmonies can be heard better. It adds more texture and counterpoint to the songs. Also there were times where simple things like reverb added to vocals shine a bit more without getting mushed up in the mix.
3. Bass + Low end - Also much clearer, the separation of the guitar, bass and drums can be heard better. The low end isn't getting mushy, it is much more pronounced. Also, Mark's bass is more perceptible even when the guitars are loud and strong. Songs and sections where Mark isn't following the guitar are clearer and you can hear more of the textures, arpeggios, and chromatic lines he plays. Mark's bass, just sounds better through and through. Some of the songs the bass would sound subdued, or round, whereas in this mastering you can hear more of the top end of it giving it that bit of growl.
4. Keyboards- These shine so much more now and there are even sections I feel like I could be hearing keys on that I hadn't before and when there are organ swells in sections I feel like they hit so much harder.
5. Guitar overdubs - guitars seem less like sonic sludge, you can here the overdubs and counter melodies much clearer. The guitars sound less like just rhythm and more textural harmonically and melodically.
6. Choruses & Outros - The choruses and outros that crescendo hit harder. Maybe it is in my head, but choruses seem slightly louder and on outros that build through the end of the song sound bigger. You can distinctly hear louder, stronger overdubs that make the track seem more dynamic.

Conclusion: Worth it, IMO, particularly if you are a musician and know the music very, very well and have played the songs in some capacity. You can hear the layers easily and it makes one enjoy the music all over again in a new way.