Although UV filters are controversial in digital photography, I feel it but to protect useful to the own optics against trapped dust, sand, mist or similar environmental influences. If you spend several hundred to a thousand euros for his lens should then however not save the UV filter, because the image performance is only as good as its weakest link - and, unfortunately, an inferior UV filter is too often the latter is filtered to B & W and. Hoya I've never had problems and so I had no difficulty choosing.
The filter has virtually no negative impact on the optical imaging performance, at least on my Sigma 50mm f / 1.4 EX DG. The protection against dust inclusions is good and the cleaning of the filter with the appropriate brushes / compressed air very easy. A contrast to the previous "not nano" version so far I could not identify.
Conclusion: It is cheaper, but not cheap. Cheap filters on expensive lenses? Probably not!