To provide some kind of resolution:
After waiting far longer than was reasonable, my Sony MDR 7520s arrived last Friday. I used them all week, for several hours a day. Mostly Cocteau Twins on tap, with a bit of modern Kate Bush for good measure, so not exactly a huge variety of musical styles. The short version is that I am satisfied for what I paid (less than half of typical), but would not be at full price. They are not perfect for my use case, but they are sufficient and I essentially like them.
They sound reasonably neutral and are very clear. The bass is stronger than I'm used to in headphones, but it isn't overbearing and I suspect it is actually more accurate when you bear in mind the fundamental limitations on bass in headphones. The sound is quite sensitive to exactly where I place them on my head, but they do stay where I put them so this isn't an issue.
They are very comfortable, with the caveat that my ears touch the inner plastic which can sometimes be irritating. They are very easy to put on and take off quickly, critical in my environment. I am told by the person who sits next to me that they don't leak any noise. Unfortunately they aren't especially good at blocking external sounds when no music is playing. They take the edge off, nothing more. Fortunately I can keep the music loud enough to block out conversations, an advantage of the low-leakage. Finally, they feel extremely well-built, despite the construction being all-plastic.
Maybe one day I will have some AKGs and Shures for comparison, but I suspect it won't be any time soon. I was very satisfied paying $350 CAD for my Etymotic ER4s, and I am basically satisfied paying $200 for these. If I had paid the $500 asking price, I would feel that I had made an error. That said, I am judging these based on my use-case; I do not feel qualified to assess them against their retail price if they are being bought for more general usage.
EDIT: After more than six months of ownership, these headphones have really grown on me. I think that speaks very well for them, since the usual trajectory is for people to be less in-love with audio equipment over time.
