I'd like to second Demera as among the best Ethiopian spots, if not the best. I was there for the first time a week ago. Two of us shared a vegetarian sampler (you pick your own dishes for it) and a meat sampler (again, you pick your own dishes for it). Dishes were nicely spiced and flavorful; the injera bread was the best I've had. Comfortable setting and good service.
Ethiopian Diamond was my longtime standby, and I still think they do many things better: quosta is usually amazing, doro tibs watt is superb, and it's nice to have that bone-free chicken alternative there (far superior to the doro tibs at Demera). However, the new place, Demera, has the edge on yemisir watt (generally my favorite dish) and shiro. I've only been to Demera once, and Diamond scores of times, but when I get back to Chicago, I want to hit up Demera again. Ras Dashen is my least favorite of the three, but it has some interesting and yummy veggie dishes (okra, mushrooms) that you can't get at the others.
I've been to three Ethiopian restaurants on the far north side, and my favorite is Ethiopian Diamond at 6120 N Broadway St, Chicago, IL 60660 (773) 338-6100. It is not an upscale kind of place, but it is comfortable, the decor and lighting are pleasant and unique, and even if it is crowded, the service is usually attentive. They often have live music which ranges from fun to ... bearable.
http://www.ethiopiandiamondcuisine.com/ (you have to visit the website with Explorer)