I stupidly have used both Purple Parking and APH at Gatwick.
Having learnt my lesson, I now use the long stay at Gatwick. Far easier, we tend to drop everyone and bags off at the drop off, go park, get the bus (and not have to lug suitcases on), and find the rest of the gang now nearer the front of the check in queue.
On return, we go and grab the coach, get the car, pick everyone else up and all the luggage from the drop-off point and be long gone within the claimed "20 minutes" its takes for the offsite coaches to arrive. "20 minutes" obviously meaning up to 90 mins, although usually nearer to 30-40 mins.
For those going from Luton, using the train station carpark at Luton Parkway is much cheaper than the horrifically priced onsite, but you need to get there in plenty of time due to the incompetence of those designing the road layout leading to the airport - even the buses get stuck in the 45-60min delays.