These Jazz Bass replacement pickups are humbuckers that feature two coils side by side, similar to a split P bass pickup. They are loud and powerful with a different tone than single coils. Not a bad sound, but not what you'd expect from a typical Jazz bass.
Pros - Well made, dead silent and very thick and even sounding.
Cons - May not have the brightness that many modern and slap players prefer.
Dimarzio Model J pups have been sold for well over 30 years and their continued sales show the utility of the design. Fundamentally, they improve/modify 4 aspects of the classic Fender Jazz pickup design.
-Due to a split-coil humbucking design (much like a P-bass pickup) they are noiseless when soloed or in combination.
-Individually adjustable poles allow for more even string to string response.
-Generally stronger output than stock pickups
-Puts in a bit of the mids and lows that a traditional Jazz set lacks in certain settings.
I installed them in a parts bass that had formerly worn a set of Fender "Noiseless" pickups until one of them died. I found that the Model J's did all the things mentioned above well. The tone is balanced and thick and there is no hum in any position. I found it to be notably warmer, thicker and less "zingy" than a traditional J pickup which was just what I was hoping for.
I'm generally a P-bass fan so while Iand prefer a thicker sound I really don't want to loose clarity and these were perfect. It is also great to be able to solo either pickup in any situation and not have to worry about single-coil buzz. Some folks have compared model J pickups to having a pair of P-pickups. I wouldn't go that far, but there are some P'ish characteristics in a model J.
It must be noted that Folks looking for a very lively bright set of pickups will probably not find these to their liking. The less-emphasized highs of the model J means that such a player would probably be better served by the Dimarzio "UltraJazz" set which has much stronger highs and a brighter voicing. The Dimarzio model J would be fine for finger style and pick players, but slap players would probably benefit from a brighter pickup as well.
As an aside, I installed these with a series-paralell switch and I can report that despite not being single coil pickups, switching to "Series" still brings the thick humbucker'ish tone that one would expect. The differences between the two settings aren't quite as pronounced as would be with a true single coil set, but they are definitely different enough that a series parallel switch is a worthwhile addition to a bass with these pickups.
In conclusion, I've had these pickups for about 4 years and can confidently say that, if you're looking for a bright and modern pickup this set not for you. However if you want a J pickup set that is a bit thicker and rounder than a traditional J with even response and a strong (but not overdriven) output, this could be exactly what you're looking for. It's a time tested design with a great tone that is entirely worthy of consideration.