Summary: I'm surprised that it actually can improve your photos
Comment: There is a lot of controversy about whether or not a UV filter makes your photos better or worse. The manufacturers claim that it "removes haze" and purists say it degrades your photos. I was very skeptical because another piece of glass between you and the subject has to be a risky proposition image quality-wise, but I was very interested in protecting my lens investment. I chose this filter for my Sigma 18-125mm OS lens because it seemed to be about midrange quality, as I suppose the lens is, and Hoya seems well established.
As I often do, I did a number of test shots and to my surprise, on a sunny day at longer focal lengths the photos were unmistakably sharper with the filter than without it. This was not true at 18 mm where it was fuzzier with the filter, but this difference all but vanishes when stopped down to f/11. (with or without the filter, the lens appears to work best at f/11.) In some shots it appeared to make the color of grass a little more naturally green.
I haven't read a scientific discussion about why these improvements might be happening, but it makes sense that if you prevent UV from entering the lens barrel, it will not be able to randomly bounce around between all the glass elements inside. I recall from basic science that light at the high end of the spectrum such as UV light tends to scatter much more than longer wavelength light such as visible light.
So for now I'm leaving it on, remembering to take it off for wide shots with wider apertures.