roomkey.com is a little unique because you can compare most major brands right next to each other. You can sort by distance or price, so it's easy to see if that chain which offers free breakfast on the weekend is really a better deal than the Marriott next door. You're eligible for any frequent stay programs because when you decide on a room to book you are taken directly to the hotel's website and book directly with the hotel -- roomkey is NOT a third-party booking site.
So don't expect unusual bargains here. In fact, just the opposite since you may have rate codes available to you which won't factor into the prices you see. But it's great for seeing what's around and getting an idea of the prices.