what makes them my favorite is the lack of sustain or the quick decay. it sounds like so many of the motown/soul records bass that i love. a lot of people will put a foam mute, i find the pyramid decay so quickly, you don't need a mute, it's built into the string. that "dead E" sound is what i love. eventually the rest of the set mellows to be the same, it just takes a little playing. In addition to that, i also love the tone, they are super heavy/thick in the bass and midrange, (especially the midrange, the mids are so beefy and fat) and light up top. as a soul/blues/jazz guy, i want all beef, no sizzle, and that's what they provide. basses i use with them have been fender jazz and hofner club bass. Labella are a nice string too, they sound great on fenders (hello jamerson, jerry jemmott, etc.), they are a classic for a reason. most unique flats are nice. TI are cool for more of a woody sound with midrange/trebly type flat sound with lighter deep bass. GHS precision flats are nice and similar to Labella just a different flavor.
the pyramids are handmade and finished, that's why you get black all over your fingers after playing them fresh out the pack. that's graphite or some other polishing compound. a week later you won't have that anymore, but it's something else unique they do that makes them smooth.