The pluses to the store are what you might expect. Lots of Patons, lots of Encore, lots of man-made fibers. Lots of Sugar & Cream. A clutch of discount & high end brand novelty yarns. But they also carry a full range of bamboo needles, magazines, and some patterns.
After browsing Bernat wool and Kroy, I was surprised to see some bamboo yarn and banana fiber yarn. Wow.
Comparing Mill End to your LYS or to Michaels or JoAnn Fabrics, I don't think the prices are significantly better. If you're going for higher end fibers, the lighting is going to be better at your LYS—it has to be!