I have one client, who, when he's on assignment at a studio, always sends me pages as he goes along and asks for my opinion. Other clients wait until something is completely finished to give it to me. I let the client gauge how much involvement I have in the development process. If a client comes to me with a script that I don't believe I can sell or won't get them work, then I tell them my thoughts on it. If they think they can sway me, I will listen.
Professionals critiquing scripts is an interesting business.
Their job is to tell you how to strengthen a script. And I don't believe they would do that. But it's very hard to get your friends to give you an honest response. I still had the same problem. When I spoke with the writer and told him, he flew off the handle and cursed at me and told me that I didn't know what I was talking about because all his friends who read the script liked it.