Down sleeping bags have the advantage of being lightweight yet wonderfully insulating. Down fill possesses a uniquely effective warmth-to-weight ratio. The deuter Astro Pro 1000 will even keep you warm down to 18 °C below freezing! A down sleeping is therefore ideal for tough mountaineering, backpacking or bike tours.
But you need to consider the climate. Down does not like moisture. If you’re going to be in damp, humid or rainy conditions, or on long winter trips, down fill can end up clumping together and losing some of its insulation effectiveness. So be careful.
A down sleeping bag is ideal for a night spent under the stars in winter in sub-zero temperatures, but also for cool summer nights in a tent.