I must say that I really don't like synthetic fibers. While wearing them, I can't get rid of a feeling that my skin doesn't breathe at all. One day I've stopped buying any clothes made of synthetic fabrics (apart from swimsuits). I still have some polyester or spandex clothes in my wardrobe but I don't have a desire to wear them very often. And when it comes to sewing, I always choose 100% or at least 80% natural fibers. Why? 

Man-made fibers aren't were breathable. This factor may cause skin rash, allergy, hyperhidrosis, etc. While being manufactured, synthetic fibers are treated with a variety of different chemicals. They can also cause unpleasant and harmful effects. The worst fabrics are those that are wrinkle-free, moth repellent or stain resistent. 

Unfortunately, synthetic fibers cost less than most of the natural fabrics. But even though pure cashmere or luxurious silk can be very expensive, it's always possible to find cheap natural fibers. Especially on seasonal sales! 

Sometimes it's too difficult to stay away from synthetic fibers. For example, 100% natural fabric isn't the perfect choice for a classical pencil skirt or a jacket. Unavoidable wrinkles won't add any charm to those garments. In this case it's not a bad idea to use a blend of natural fibers with 20-30% of synthetics. 

Some people believe that synthetic fibers last forever. I can't agree with that. All my polyester dresses were loosing it's shap and were looking really bad in general after washing. But as for the natural fibers, I still have some vintage silk dresses that look great, even after all the numerous washings. It proves that natural fibers can be a better investment. All you have to do is to check the fabric quality before buying it. Usually it's quite easy to see if it's a good quality or not.