My first instinct is to tell each reader who gave me some negative feedback to go write their own screenplay and let me read their work so I can give them negative feedback.
Yeah, take that evil readers!
But my evil readers explain to me that they don't know how to write, that it was me who asked them for the reading favor in the first place, and that they are just stating their honest opinions and I should really just chill out or else they will never read one of my screenplays ever again.
Hmm... Sigh... My evil readers (sometimes friends) are right.
It's not a reader's job to know how to write, btw, it's a reader's job to know what is working and not working in a screenplay. Great screenplay readers read a lot of fiction books, screenplays, and watch a lot of movies. Great screenwriters write, REWRITE, and then rewrite some more without complaining.
That said, how do you know if the feedback you're getting is any good? After all, you thought your screenplay was brilliant until you heard the feedback.
My rule on feedback is this: If two readers have identical problems with your screenplay, you should feel confident you need to fix the problems.
And that's it. That's how you know whether or not you should listen to the feedback those reader dudes and dudettes are telling you.
Two more tips: always try and get three readers to give you feedback. Also, I would try to choose readers who have no ambition of becoming a screenwriter.
I've met too many reader/wannabe writers who are so jaded and frustrated with their own scripts that they sometimes will destroy a new writer's ego to make themselves feel better about their own writing ability. (There are exceptions, of course.)
Instead of risking permanent emotional damage from some bitter wannabe writer, look for wannabe or established (if you're lucky) producers and directors, or aspiring screenwriting teachers to give you feedback. Trust me on this one. You want to be very selective about who you choose to read your screenplay.
Okay, I'm done. Keep rewriting and maintain an open mind with the feedback you hear. That's how screenplays become great!