Polarizer & ND filters for my Nikon 17-55 f/2.8

I'm looking for a circular polarizer and an ND filter for my lens.  I checked amazon.com and there are filters from Vivitar, Polaroid and all sorts of third-party brands which are SIGNIFICANTLY cheaper than the Hoya/Tiffen/B+W counterparts.  I understand you get what you pay for, but I really have ask whether the B+W $100 filter is proportionally better than a Vivitar for $15-20.  Is there a serious degradation in image quality? Is it OK to buy the third-party filters?

Also, do graduated ND filters reduce sharpness more than just using a fixed ND filter?