I've flown to Tampa via Charlotte. It was really easy - you go through immigration at Charlotte so by the time you get to your final destination there's no queue.
Definitely recommend having a longer layover (as in a couple of hours). Bear in mind you have to factor in getting off the plane, immigration, getting your bags, putting your bags back on the transfer carousel etc it takes a bit of time. Some website offer an hour or so layover which is not really enough time if it's busy. There are a few bars, restaurants and shops at Charlotte so better to take your time and relax! Having said that, my parents missed their connection because the plane was late and US Airways immediately put them on the next flight.
Hope that helps