I've always heard the bison ribeyes are the bomb... never managed to find any at the local market, though. Going to have to try again this summer.
If you find them as mentioned before proper cooking is the key. Sear them over coals for about 2 minutes over really high heat, flip and do the other side, reduce the heat (indirect method works well) and cook to medium rare at most. One of the best steaks on earth. Taste and smell is identical to beef to me, but after reading up on the nutritional stuff I'm convinced God put them here for us to eat. If you like your beef cooked well Bison may not be for you. It's just not as tender that way, but cooked properly it's every bit as good as beef.