It isn't used by the vast majority of the ecosystem, but Const is in very active use.
The only major consumer of Constant at this time I think is Rusell O'Connor's multiplate library.
On a related note Simon recently asked the core libraries committee if we should move Data.Functor.Identity into base.
I'm very strongly inclined to say yes, as was the rest of the committee. Do you have any objection there?
Between moving Identity into base and removing Constant it would mean that transformers would then only have to contain ...well.. transformers. =)
-Edward