Adder, I agree some aftermarket products will devalue a car more as it can be an "unknown quantity", but it will depend on the level of aftermarket add-ons. I had no problems selling mine with Eibachs on, but then that was the only modifacation I made (and was well worth the money IMO). But at the end of the day, money saved is money saved and you cant argue with that, especially in my case and many others, a saving of over £2000 and a reduction of a £100 over a UK car at resale.
In the case of imported 172s, the differences compared to a UK car with insurance, warrenty, servicing/parts costs etc is negligible, and I think youll find that 9 out of 10 people would rather save money on purchase and worry about resale value later.
IMO, Imports and car supermarkets are great creating competition and driving prices down. More and more people are realising the saving to be had and its only UK dealers who have sour grapes as they are loosing sales to these supermarkets. About time too as generally its regarded that the UK are over charged massively compared to the rest of Europe. UK specd cars are no longer the "holy grail" as dealers make out to be and will do anything to get you to buy a UK car and thus its the UK dealers who have let the "cheese smelling" go to their heads with their cheesy sales pitch.