Currently evaluating Derby Extra, Premium and Usta. The Extra and Premium are a lot like Astra SP's. Smooth shave and I think they give up a closer shave on my usual 2 passes. Trying to finish the evaluations of all three before buying 100, almost pulled the trigger on the Extras though. Quite a surprise. One blade I did purchase 100 of was the Treet "Yellow" Carbon Steel blade.
OP; @dmshaver is the curator of the Great Blade Exchange. Very much worth the time to swap and sample if you haven't already.
Have fun and as always YMMV, especially with blades!
Derby has up'ed their game! Thanks for the post. I have used all three and find the Premium's to be the best, but all of them are decent or better. Astra SP to me are ok. I prefer the Astra SS.