I can't say I've given it too much of a beating yet, but the DX-40 is thus far quite impressive. My one issue with VX-21 was it's so-so resistance to point abrasion (sharp rocks and ski edges from the outside, carelessly packed paddle shafts from the inside). DX-40 seems to take care of that concern with ease.
I've made two refinements to the original pack. The first is that I relocate the internal compression strap attachment point on the back (not against ones back) of the pack down abut 6 inches. The second is that I ripped open the side panel-front panel junctions and drastically altered the contour of the side panels.
The result of my original taper is that (on me) the back panel sits close to vertically, and thus the upper section leans backwards when the pack is full. No good. In the revision I cut the side panels so that the bottom section is parallel with the upper most seam, but the side panel tapers 2 inches wider over about six inches, with the shoulder strap attachment point being right in the middle of that six inch span. So when viewed from the side the part against my back goes straight up, jogs inwards towards my ear following the curve of my shoulder, then goes straight up again.
The improvement in load carry this makes is quite drastic.