I can still smell the tyres and engine of the Accord demonstrator after my Dad and me had finished "test driving" it.
A demonstrator has to be tested properly as you test drive it for 10/20 minutes but could end up making a expensive mistake if not tested well.
So no I wouldn't buy a demonstrator!
My previous Civic was a pre reg which was sat in the showroom for a few months. So still got a good deal and it had 9 miles on the clock when I picked it up
