I have a number, both old and new. Fit and finish on the new ones surpasses what they were doing in the late 90s and early part of this decade, and also the transition to slightly bigger handles and more meaty guards (longer, thicker) has been, for me, a welcome one.
I don't think you'll easily find a difference, use-wise, between the old 52100 blades and the newer ones in 0170-6. Marble's is running them at very similar hardness and the characteristics of the two steels are not that different. I have some custom and high-end production (Swamp Rat) knives in 52100 that pull more out of it, performance-wise, than Marble's did. Nonetheless, either makes a great general purpose blade---easy to work with, will support the thin edges that come on these knives and will hold that edge well in a wide variety of cutting tasks. Most of the complaints against the new steel were back when Marble's first changed it and were calling it "Alchemite steel" or something very made-up sounding, which made people a tad nervous. Once they owned up to what it actually was, most of the cries for TRADITION!! kinda fell by the wayside. Of course, it's interesting now that Camillus (who was making Marble's blades for them) has gone under, what new steel will have to be moved to. Honestly, I think they'd do fine switching back to 52100, or if they're wanting to go cheaper just 1095--a great steel when done correctly.
As to sharpening, I use sandpaper for convex sharpening with a hard rubber backing (mousepads have too much give for my taste--tend to round my edges off) and whatever level of grit is necessary depending on how much metal I'm looking to move---just like when choosing the grit of a stone. For serious removal, to reset an edge or reprofile the blade, you'll probably want to start in the 150 or 220 range. You keep working up from there--I don't like to skip more than about 200 grit at a time or else you end up doing a LOT of work with the higher grits to see results. If you're just doing normal maintanence sharpening, then it depends on your edge preferences. I like a toothy edge for skinning, so I probably wouldn't go much higher than 600 or 800 grit. If you want finer, you can go all the way up to 2000 grit with the paper offered in most automotive supply stores, and far higher than that with a little searching on line. My choppers tend to have final edge polishes of at least 1500 grit.
Just like with stones, it's not usually necessary to sharpen all that often if you just maintain your edge a bit. I can wash off a knife that I've just been skinning with, strop it a few times on some 600 paper and be ready to go again--adjusting your pressure can let you hone versus actually removing metal (or at least a significant amount).
This ended up longer than I'd intended. I'd say get some 220, 400, 600 and 800 and experiment. You may find that you have all the grits you need. If not, then get back to that store.
