I need a Y/C comb filter for my D503 player. I found a KRAMER FC-100 one, does anyone have experience with this? I need to know if it's a good one before I buy.

No it's not good. Your bare minimum should be a faroudja nrs with its superb 2d-4line philips comb filter and faroudja patented cross color suppression. Best is motion adaptive 3d comb filtering found in devices between 2001-2006. Modern tvs upto 2011 had 3d comb filters built in. Some dvd/vcr etc recorders also had 3d comb filters which you can use if you pass your signal through them. None of the video processors other than real expensive ones had 3d comb filters. I never ventured into dvd vcr route. Perhaps someone else can tell you some worthwhile makes/models.

Lot's of info in this thread:
http://forum.lddb.com/viewtopic.php?f=27&t=562

I'm using the Panasonic DMR-ES15 DVD-R and love it.

If you want a Faroudja VP-100, I have one for sale.