I used them in Fipp to replace Reducers:

https://github.com/brandonbloom/fipp/commit/b83dfb3b3ac7c90d...

My use cases are 1) to emulate yield / a pipeline of generators 2) to avoid intermediate object creation. My code is an indirect port of some Haskell code, so it's a tad awkward, but overall Transducers were a small perf win over reducers (which was a big perf win over lazy seqs) and also provide a good effectful-ish model for my pipeline.