While evaluating these blades, I used them exclusively for 2 weeks and only in a Merkur HD. The first week was with Taylor's Avocado, and the second week was split between Taylor's Rose and Proraso. For the sake of argument, one may claim that these blades perform differently when used in a different razor or with a soap instead of a cream.
Price - If purchased through on-line auction, these blades can be anywhere from 15 to 25 cents a blade, but you must buy 100 blades. If purchased from Derby's website (not sure on minimum order size, but I believe it is the neighborhood of 1000 blades) then you could get a greater discount. If you choose to purchase these from a vendor (I have only seen one who sells them) then you could pay as much as $4 for a 5-pack. My recommendation would be to purchase through auction and if you don't like them, sell them.
Sharpness - While this is a very subjective area that many gents would be willing to argue about, I personally find these blades to be as sharp or sharper than any blade on the market except for maybe the feather blades. While I believe the feather blades to be sharper, I find that the Derby's lack any of the harshness/volatility that I experienced with the Feather's.
Longevity- I get 7 wonderful shaves from these blades. I really think that I could squeeze out a few more shaves, but at 15 cents a pop, its not really worth the risk.
Smoothness of Shave - These blades shave me as smoothly as any that I have used and without irritation.
Packaging - Derby's come in the same standard plastic packaging that Merkur's and Feather's come in with the one difference being that they are packaged in 5's instead of 10's. While there is nothing wrong or inconvenient with this method, I rank packaging at an 8 because (for all 3 companies) I feel this is an area currently being overlooked in the shaving industry.
Quality - The overall quality of these blades is fantastic. They provide the kind of shave that we, as men, should demand from our blades. The only thing that prevents me from making a bulk purchase of 1000+ blades is the fact that in the next six months there will probably be 2 or 3 more brands of blades surface in this forum, and I would like to be able to purchase them without arguing with my wife.
***There has been a lot of talk about the lack of a used blade slot with the Derby's. It does have one, as seen in the picture below.