Having tried nearly every manufaturer of pickups (including custom wound Seymour Duncan and Lindy Fralin), and also having way too many BK pickups for my own good, I can say that I can hear the difference that BK's make in my guitars.
However, I still have some SD's (though they are very old ones, or wound by Seymour in the custom shop), some Fralins (which were also custom wound), some original D'Armond, Daneletro, Fender and Gibson and yes some EMG's.
Basically I have not replaced everything, but whenever I compare the BK's to the others, I end up ordering more BK's! The worst example of this is my PRS's. I had very old 80's pickups in my Standard and EG, and early 90's artist pickups in my Artist II. I liked the 80's pickups much more than the 90's, and also wanted to try a Crawler on my EG. So I got a Crawler and 2 P-90 set for the EG. They sounded so good that I got a VHII set for my Artist, which transformed it. The problem was then that it sounded so much better than the standard with the 80's pickups! I finally put a set of Rebel Yells into the standard last November, and now it sounds great again.
The BK's raise your tonal expectations and then other pickups don't sound as good. But there are horses for courses, and sometimes I want to have that EMG sound, and I love the original JB (which is nothing like the new ones). Also my 62 tele sounds really sweet and I wouldn't change the sound of it at all.
So it is all down to what you want and what your ears tell you. I know that the d'Armonds are the pickups I would pick for that old rockabilly and blues sound (because that is what was used anyway), and BK don't make a pickup like it, but if they did I am also sure that it would sound better than the new d'Armonds because it would be hand wound like the older ones.
