You've spent weeks writing your latest draft, tightened the structure, nailed the characters, hit the arcs, refined the dialogue and you're finally happy with your screenplay... time to send it out.
Notes, feedback, or whatever you want to call them, are vital before sending your script out to producers, directors or agents. It doesn't matter how many years you've been writing, how far you are along with your career, everyone needs feedback on their work... EVERYONE!
The main reason for this is what I call 'Shite Blindness'. When you've been working on a screenplay for so long, reviewing, rewriting and working through several drafts, you are going to be too close to your work to be objective enough to see what's wrong with it and make the hard decisions. You might have an inkling something is wrong and not be able to put your finger on it. You might be completely blind to the screenplay's obvious faults. What you really need is an objective opinion on your work. And not just one persons, but several if you can.
I would recommened sending your screenplay out to at least two professional readers and three peers, for their thoughts. It's always better to have a fresh perspective on your screenplay than to go it alone and realise you have a gaping hole in your plot, just after you've pressed send on that email. If you don't actively seek feedback to help improve your screenplay, the producers, directors and agents you send your work to are going to notice its faults and you're just giving them an easy excuse to say no to you and your work. Get those problems with your screenplay sorted first, then send it out knowing it really is the best it can be.
Don't be tempted to rewrite your work while you're waiting for those notes. If you think of something make a note of it for later. Wait until everyone has come back to you and then read through those notes in one sitting. Leave them alone for a day. Cogitate on them. Then come back and read them once more.
Now's the time to compare each set of notes. If more than one person makes the same point, or mentions the same problem, then you can be sure that point is something you need to look at closely and deal with. There will be other issues raised, but if only one person mentions them you don't necissarily need to change them. It's up to you if you choose to or not. The only concerns you must address are the ones mentioned by more than one person.