So far, it has been a very good UV/protection filter. I don't notice any difference in sharpness when I put it on, and love the secure feeling that the lens front is protected from dust and moisture.
After using this at +0.3 f/stops with velvia 50, I discovered that the film was underexposed.
I then did some resting with my digital camera. Discovered that it was in fact +1.0 extra stops.
Even so, it is still the brightest CPL I have used.
This filter works as advertised and my photos look incredible..this was an expensive purchase obviously, but well worth the peace and mind knowing that this will work and work consistently for a long time.