Gatwick is a very good airport. You can fly non-stop to more locations in the US from Gatwick than you can from Heathrow. Many of the flights from London to South Africa leave only from Gatwick. Gatwick still has a fair number of charters (most leave from the North Terminal) but many charters have migrated out to Lutton and Stanstead airports. The car rental lots are located on site at Gatwick and thus save you time--at Heathrow you have to wait for a van/bus to take your car. Heathrow and Gatwick both have express trains into London. If you want to earn more miles, the routing from Dallas to Gatwick on AA is longer than the routing from Chicago.