Here's my thinking, having guitars with all three pickups: The 57/08s are a warm pickup with a fatter lower midrange than the 85/15s (or one might simply say that the frequency balance tends to favor the lower mids more). But there's also some cut in just the right part of the frequency response that's right where it needs to be to keep the pickups from sounding muddy.
I have a low-turn version of 57/08s in my McCarty Singlecut, but have had the other versions in former PRSes. Being something of a traditionalist, I tend to favor these LT versions.
The 85/15s are hotter, and have a balance that favors the upper mids and higher frequencies. They're also clearer than the 57/08s. They're one of the things that make the CU24 so useful to me, since I already had 57/08s in other guitars. I really like the way they work for solos that have to cut through a dense mix - I tend to need that with some of the styles clients have me do in my ad work.
The 58/15s remind me of a cross between the 57/08 and a Soapbar pickup. There's extension in the high frequencies, great clarity, and they split incredibly well. Again, there's tremendous utility in having my McCarty, a guitar that sounds different from the other two. I really like this guitar for rhythm work, arpeggiated chords, and soloing if the mix isn't too dense.
I find that every guitar has its own unique purpose in my productions.
I'll also put in a good word for the pickups on my 20th PS Anniversary guitar, the Paul's pickups with the Narrowfield middle pickup. This guitar is so gorgeous sounding that it's almost a shame to use it in a dense rock track, it sounds so good in a smaller ensemble! It's clear to me that PRS has been thinking a LOT about pickups and how they sound best in various instruments. This is such a good thing!
I'm looking forward to the 594 with the LT version of the 58/15s.