Shingle Underlayment

There are so many products on the market these days, from your basic 15# felt to all of the synthetic materials. Just wondering what some of you guys are using. I still prefer the 15# because I don't see that great of an advantage to all the newer pricier products My take is that once you have roofed over it you have thousands of puncture holes so what is the advantage to spending more money on underlayment. Any thoughts or strong opinions? Lets fight about it and bring this place back to its former glory! :laugh: