BMW are worse. The 5-series has about the same depreciation and around the same list price. as the GS. However you need to add options on the BMW (steering wheel, seats etc) which add £1,000s to the cost only for the dealers to offer you standard spec money when you trade in!
Depreciation is always heavy on large cars and it's something you need to consider when buying that type of car, but it's the mark up that gets me. Gary's had the car a few weeks, it's had all the problems sorted out, in showroom condition so the dealer has to do nothing and they will put it up for £15,000, maybe £14,500 as it has another owner!