I have been using Prismacolor pencils for a good amount of time now. They are great for layering color. They can help you put more precise details into your paintings too. I wait until a painting is dry and then I can add some detail whith a well sharpened pencil. I have never used Polychromos, but I ran across this great product review about Prismacolor vs. Polychromos: http://liannewilliams.com/sketchblog/2015/5/26/prismacolor-vs-polychromos-lets-compare
Lianne Williams does such a good job at showing the pros and cons of both pencils. She shows good comparison pictures of using both in different drawing techniques. I definitely would love to try both pencils in my art some time after reading her product review.