This needs to be clear--when you ask a professional for their take on your material, you're not just asking them to take an hour or two out of their life, you're asking them to give you--gratis--the acquired knowledge, insight, and skill of years of work. It is no different than asking your friend the house painter to paint your living room during his off hours.

Honestly, well worth the read. The post goes beyond just screenwriters and talks to any professional who's acquired years worth of knowledge and skills.