Installed it on my M4 BP while I was CLA-ing it. The result was better contrast and less flaring, more inline with a coated VF like MP-2. Well worth the $90 IMO. Have not found cleaning a problem, I just use microfiber and fingerprints, etc. - come clean off.
Leica: M4, M4 BP, M6 ti, 21/3.4 v1, 28/2.8 v4, 35/2.0 v1, 50/2.0 v2, 50/2.8 v2, Elmarit-M 90/2.8
Cosina "Voigtlaender": 35/1.2 v1