The main downside is a lack of choice in terms of colour/general spec etc, but if you're happy with how the demo model in question is configured, this is a moot point. There's also the wear on the car to consider, but this is likely to be minimal. It's a good way to save money on a car and to avoid some of the initial depreciation. It's also worth looking at pre-registered examples of the Nissan Qashqai which won't have been used at all, but could offer similar discounts.