its basically a humbucker though isn't it? it just fits in where a single coil would? or do they actually sound like single coils...?
It's technically a humbucker, but because the coils are stacked and the polepieces are alnico rod magnets, the construction is very similar to a single-coil.
The older ones (Duncan's Stack series and DiMarzio's HS series) don't sound very much like true SCs - the Duncans in particular are a bit thin and lifeless. The newer ones (like Kinmans and DiMarzio's Virtual Vintage series) are more sophisticated - I can't claim to understand the science, but they've had all sorts of design tweaks so the bottom coils are there
just to cancel hum, without affecting the single-coil tone much at all.
Just to confuse things, you can also get single-coil sized humbuckers with side-by-side coils rather than stacked - so they're constructed like full-size HBs. Some are intended to sound like SCs (Duncan Duckbuckers, DiMarzio Cruisers, Joe Bardens), others are just reduced-size HBs for Strat and Tele owners who want HB tones without carving up their guitars (Duncan Hot Rails, Little '59 and JB Jr., DiMarzio Fast Tracks).
True single-coil tone connoisseurs will say that none of these sound like real SCs, and anyway the hum is part of the character of single-coils. I'm sure they're 100% correct. But for someone like me who uses mainly humbuckers, the Kinmans and Virtual Vintages are close enough for rock'n'roll. I'm not that bothered if a pickup sounds exactly like a '54 Strat, I'm happy if it sounds
good. And doesn't hum. But that's just a personal opinion.
One more thing - most of the Kinman and Virtual Vintage pickups are designed to sound like variations of '50s or '60s Fender pickups.
You don't get the huge choice of single-coil tones that are available in the BKP range (just remembering where we are

). :D