I have the 112mm circular polarizer from NiSi. I also have a square 100mm polarizer and a whole set of ND NiSi 100mm sqaure filters as well and they all perform excellently. I purchased the 112mm circular polarizer as a bundle with the 112mm NiSi metal lens hood as it was a cheaper alternative at the time. The only trouble is, I would only ever use the circular polarizer on the 14-24 for waterfalls etc to reduce reflections on leaves, I would never use it where a full sky is involved due to gradual polarization across the sky that occurs on wide angle lenses fo greater than 35mm. So, I have not used the NiSi version for waterfalls as I usually have my square filter set on me as I may want to use ND's as well. If I knew I was only going to need the 112mm polarizer and not the ND's, then I would try it out but that hasn't been the case yet. I really want a 3 stop, 6 stop and 10 stop polarizer or a variable ND/polarizer 112mm.
My round filters are all Breakthrough and they are also excellent.