Quite commonly this subject comes up, and there are those who say don't use them, and those that say do... The arguments are that putting a cheaper piece of glass in front of a high quality lens is pointless, and can reduce image quality. On the other hand having some protection for the lens helps save from accidental damage and costly repairs. Both arguments have merit, and I would like to share my findings with 2 Hoya UV SMC filters.
These are high quality filters, but non the less caused severe image degradation. In the photo below(Click for larger view), the shot on the left has no filter in place, the one on the right has the Hoya filter in place and you can clearly see the difference and lack of contrast and sharpness. I was happy with Hoya's customer service here in Canada who gladly without question and without any receipt took back the 2 defective filters and replaced them free of charge. However, I am now using B+W Multicoat UV's on my lenses, and so far there has been no affect on image quality, and I am very pleased with the results of these rather expensive filters... Nobody said photography was cheap!! LOL!
My conclusion is simple, filters can affect image quality, not every filter will, so my advice is to test your gear when adding something like a filter, because it could mean the difference between a good photograph, and a great one !!! Don't cheap out when purchasing filters, it makes no sense what's so ever to put a $10 piece of glass in front of a $1500 lens, buy the best you can afford, name brands like B+W and even Hoya are good ones... Most importantly, test them!! Pick a subject to shoot on a tripod, and take shots with and without filters and review the results on your computer at higher magnifications... The sooner you can identify any issues the better!!