'Two hundred and fifty quid for a sleeping bag?!' I hear you cry. 'For that, I'd expect it to pilot a fighter jet, tapdance and play chess! What could possibly make a humble sleeping bag worth that kind of money?'

The short answer is simple: this is a lightweight bag, stuffed with down which makes it pricier. At 900g, it's not one of the very lightest on the market (there are several now that weigh in at under 500g, including the Rab Q-top that weighs 454g), but it does impress with a three season comfort rating that extends down to around -7 degrees celcius, making it suitable for all but the most extreme conditions you could encounter in the British Isles. This makes it a good compromise in the weight-for-warmth stakes, and well worth considering for anyone who is a keen backpacker and wishes to camp at times of year that lie outside of the summer months.

The outside of the bag made of a special Pertex Quantum fabric, which weighs just 30g a square metre, yet offers tremendous toughness and endurance (not to mention an easy-to-wipe surface, which is a must when you're outdoors). The only thing you do have to watch is that you don't catch the fabric in the bag's zipper as you get in and out, as this could damage it.

The bag is then crammed with 400g of goose down from Poland (hence the name Quantum 400). And when I say 'crammed' I mean absolutely jammed - the fill power of this down is around 750+, making the bags both soft and warm. This is what gives the bag its impressive three-season capacity at this weight: sometimes nature still does better than mankind, and down is still vastly superior to any of the artificial, manmade fillers on the market in terms of warmth-weight ratios. It also makes the bag highly compact - it can pack down to just 15cm x 25cm, which means more space for other kit in your rucksack. What's more, it comes with a lightweight stuff sack for this very purpose, as well as a larger cotton bag for storage (you shouldn't store down bags fully compressed, as this damages the filling, making it less warm).

The construction of the bag is a narrow precurved box wall, which is lighter than the old-fashioned shell construction. It has a shoulder baffle, and a full length zip, with a shoulder and hood section for extra warmth around your head. Design-wise, it's very comfortable, with cleverly spaced baffles that keep the down evenly spaced, and ensuring that there are no cold spots. I find it very comfortable to sleep in too, offering plenty of cushioning from the rough ground!

Another good feature of Rab bags is that they don't just come in one size. You can get a ladies version, an extralong version, an extra wide version and extra short version, ensuring that everyone has a bag that is comfortable for their bodyshape, while those of us that are smaller and female don't end up carrying unnecessary extra weight!

There are some negatives to this product, though. As I already mentioned, it is expensive, and many may feel that cheaper, slightly heavier bags offer better value. For those truly concerned with lightness, there are bags that weigh in at around half the Rab Quantum 400's weight, which might make serious backpackers, climbers and mountain marathoners think twice. This product is not the most pared-down sleeping sack you'll find: it still has features like a full-length zip that you wouldn't find on a serious lightweight sack where every detail is designed to minimize heaviness. Finally, if you're really worried about warmth, there are winter bags that deliver more toastiness: I wouldn't want to risk this bag in conditions that I knew would fall to minus five or below.

The main disadvantage, though, for those who camp regularly is the problem with washing down bags in an ordinary washing machine. Goose down doesn't react well to getting wet! If you're doing standard camping, and looking to spend a lot of time outdoors in the mud, a cheaper washable, synthetic sleeping bag is a much better bet.

However, for those who want to travel light, but stay warm, this bag offers a versatile compromise solution that makes it well worth considering.