Is a demonstrator worth buying?

What are the pros and cons of buying a current model demonstrator as opposed to the brand-new vehicle? I’m looking at the top-end model of a the manual petrol Nissan Qashqai.

I'm about to buy a car but the dealer won't send the documents to the DVLA for several weeks?

I recently looked at a Mazda demo car with 300 miles on it. It was available on a Mazda dealer website and I agreed a price but then was told they could let me have the car and give me a receipt but could...