I've had my GC with quadra lift for over two years and it hasn't missed a beat (the suspension that is) and I wouldn't consider buying another GC without it.
As everyone else has said it's great for making access in and out of the vehicle better for shorties, makes it easier to clean the roof and load the roof racks but it comes into it's own if you tow because of the auto-levelling and how easy it makes hitching up. With a 2t camper in tow and fully loaded jeep including a boot loaded with 40l engel, 2kva generator, portable hot water, chainsaw plus some luggage the thing sits perfectly flat.
The only thing I don't like is when it's set to ORII it tops out all the time and makes some harsh noises and thumps but in reality you rarely need this much lift as ORI gives you enough lift for most circumstances
Cheers,
Jason.