They're really good knives, especially for the prices Nathan charges.
Cutting through sand impregnated landscaping cloth quickly dulls any knife I've ever used, including my CPKs. That's a lot rougher than card board, though. The great thing is that CPKs sharpen back to a hair-shaving edge pretty easily.
Personally, I think the knives in delta 3v are almost a perfect balance between abrasion resistance, toughness, and sharpening ease.
Nathan uses the term "durable" to sum it up and I can't think of a better single term to describe the cumulative properties.