Having worked in the motor trade for the last 9 years, I've seen a huge increase in the number of people buying imported cars, and the growing volume seems to be conversely related to UK dealer's sinking tolerance of them.
At BMW (or at least at my branch), the attitude seems to be "they screwed us, so let's screw them". This can be seen in both the service department (where import drivers are treated like 2nd class citizens - paying for courtesy cars etc), and in the Sales Dept. - "This guy wants to trade-in an import, let's see just how hard we can kick it in price".
However, it is a fact (whatever Count Quent-ula claims) that the buying public expect to pay considerably less for a 2nd hand imported car than they would for a UK car. So the saving at the front end tends to come back and bite you on the arse when you sell. In saying that, if you plan to hold onto your imported car for a good few years, the pain will be somewhat lessened.
In spite of all this, I opted to buy a parallel import bike last time round - the bike market is much more accepting of the import situation (and besides, the £ balance was much smaller than with most car purchases).
The bike is great, but when servicing time came around, I couldn't get a Ducati dealer in Scotland to service it. Let's be clear, I was a PAYING customer, and they still refused to service it. They were all smart enough to say that it was a company (i.e. individual dealer) policy rather than a Ducati policy, but it still left me with a non-franchised stamp in my service book.
Things will change over the next couple of years, and, on balance, I would suggest that buying an import SHOULD be a relatively safe option. But there's a lot to be said for having the supplying dealer on your doorstep in times of difficulty...