I use the Zero for mono. On the whole, it is very good. It is the clearest cartridge I have heard. Surface noise is low, so low that a few of my records, which I had graded as VG with a stereo cartridge, had to be regraded to NM on the Zero. Also, it is a very good tracker.
But, it lacks the ultimate refinement of my higher end Koetsu - then again, it's a small fraction of the price.
I use higher end Sowter step-up transformers, with Nude Vishay loading resistors. The Zero likes a low load, about 2K on the secondary if I remember correctly.