Synthetic insulation:

Synthetic is popular because it is usually cheaper than down. Better performance in wet conditions and for users with allergies to down. Synthetic insulation is typically made from polyester, is quick drying and in wet conditions, still has some insulating properties.

There are many different forms of synthetic insulation, though the most important aspects of synthetic insulation is the thickness of the insulation and whether it’s short-staple or continuous staple insulation.

Continuous staple insulation use a thicker continuous filament that is lofty, strong and durable. Because of how they are built, the insulation stays in place reducing the risk for coldspots. However they are less compressable than short-staple insulation and feel stiffer.

Short staple insulation is often used in jackets and clothing. Instead of a long sheet of insulation, short staple feels more like down. Compresses better, softer and compress down nicely. However, they tend to leave room for cold spots and the durability is questionable.

Durability

Synthetic sleeping bags tend to break down much faster than down. Lose their loft and become useless within a few years. It’s important to store synthetic bags open and hanging to extend the life of a synthetic bag.

Compressability

Synthetic insulation tends be be bulky and less compressable. Though certain forms of insulation that have short-staple strands, can compress down more than coninuous filament insulations. They are still not comparable to down

Climate - Wet vs Dry

While not a guaranti for warmth, in fact, more often than not, having a wet synthetic sleeping bag is still going to suck, just that it will suck less than a down bag. Meaning, it’s going to be cold, but will still insulate a little better than Down. With that said, synthetic insulation does tend to dry quicker than down.