I stopped using a filter on my lenses over a year ago now and did notice a slight improvement in IQ but not a lot...only use a CPL on occasion now but have though of investing in a Cokin setup for fun, when I am "riche" that is...
I use hoods now a days and that is something I never did in the old days...if I do buy filters I buy from a company in China who typically has excellent prices and their stuff is genuine real...they are now called MaxSaver (
http://www.Maxsaver.net)...extra protection usually cannot hurt...remember the dude here who's life was saved, not to mention his gear, k7 etc...) when shooting a helicopter up-close while it was in the process of either landing or taking off, I forget...so perhaps the real reason for a filter is the save lives? hehehehe....
But I stick to better brands. Though after the "impossible to clean" issue I had with a couple Pro1 filters, later fixed with a LensPen as well as a bottle of ROR, never cared for Hoya though...I like B+W Kaesemann the best but, Kenko are just as good as any other when talking UV/protector filters (they are made by Hoya as I recall).
But stick with name brands and you'll be happy...good luck.