I've tried Contemporary (9/06), BWI (7/04), YC (1/04) and BC (8/05), the last of which was a complimentary upgrade. All had cold breakfast, snacks during the day, a hot hors d'ouerves dish or two in the evening, and desserts and cordials at night. I did not find major differences in the food offerings, but if imperfect memory helps, YC had a little better food and prettier furnishings---love that nautical thing. Contemporary by far had the best view and setting, as the lounge and its balconies overlook the Magic Kingdom from either the 12th or 14th floor. Also, Contemporary lounge was larger than the others--more room to spread out.
Service is always friendly, and staff at all of them get whatever can be had for your itinerary and so on. For some reason, BWI stood out as the friendliest, but that's based on July 2004 staff--who knows where they are today!!
Next month, we try Poly concierge in the Hawaii building. The pictures of the lounge are impressive, and I think the view will be amazing, too. We are paying a lot more for this (lagoon view, too) than other concierge experiences, probably in part because of time of year and availability of discounts. But I expect I will agree with DisneyGirlie that Poly Concierge is

!!!!