There's nothing more useful to a writer than a good critic.  Without the constructive advice of people with well-functioning "crap detectors," (as Hemingway put it) writers have to depend on their own judgement.  That's as it should be, but after weeks, maybe months of working on a project, it's hard to get the distance needed to be objective.  When you've just written something as complex as a screenplay, objectivity may be in short supply.  Your "crap detector" is only restored to its original functioning after a period of time. In other words, you can do it yourself if you're prepared to put the script in the closet for a month or two in between rewrites.

That's why screenwriters often work with partners, or hire script consultants to critique their work.  Because the critic (hopefully) has no expectations or biases, she can take an informed and objective look at your script and help you perfect it.  When it comes to my own work I'm relentless, and I put that same objectivity, instinct, experience, and knowledge to work when I consult with other writers on their projects.  I am totally honest, but I will look for the good points as well as the bad, and I will also offer suggestions on what can be done to make the story better than it is.  If you need help with a script please contact me by email.