I generally feel that if a garage treats you badly before you have even bought the car that they they don't deserve the business and also that they are going to be the last place you want to have it serviced.
I have test driven cars in the past that I then ended up buying at a different garage and it's always nice to go back and rub someones nose in it over their loss of commission.
On a related note, my little brother (24 and he isn't exactly little any more he's taller than me now) was looking at buying a z4 and several BMW garages treated him badly just because they decided he is too young to be able to afford it - he's just ended up buying a new 370z instead
