The Mag Mile Marriott is located in the River North district of Chicago, which is right downtown. Very good location for shopping and tourist attractions. To note, this is a standard Marriott hotel and not a JW Marriott branded hotel. The JW Marriott is located in the Loop (financial district) and is very nice as well.
I have stayed at the Renaissance Blackstone Hotel is extremely nice as well. It is further south from downtown but is close to Grant Park, the Art Institute and the Museum Campus.
I recently stayed at the Mag Mile Marriott and was a little let down. I was expecting more. The lobby is incredibly noisy, the rooms are fine, but not great. There's a Courtyard a few blocks away on Ontario that if you're looking to save some money would be a perfectly fine hotel (view not as impressive). It's a block from the Mag Mile.