We flew from MBJ to Atlanta and then on to Winnipeg in December. When you land in Charlotte you'll have to collect your bags and clear customs there. You'll then have to re-check your bags for the onward flight to Canada. Remember if you buy duty free in MBJ, you'll have to leave room and same some weight in you checked bag as you won't be able to pass security to make you connection.

My experience in Atlanta was great. Immigration, customs, security, and airport staff were so friendly and helpful. The Canadian system can learn a few things about customer service from our American friends.