Useful, informed, honest criticism is absolutely essential to any screenwriter. It's also incredibly difficult to find.
Most of us have an English major somewhere around us: maybe they're making your coffee, or teaching third grade, maybe even writing copy for a marketing firm. These are good, strong readers with serious attention spans and the best of intentions. Their advice can be useful, but they don't often know much about reading a screenplay. They'll frequently feel the lack of a character's internality. They'll turn your script into a novel if you let them.
Then there's the first draft issue. First drafts by definition suck. It's a given. That's why you want criticism, yo. Often you'll get a reader who can't get around that fact, and won't get to the next place: saying something useful.
Let's not forget readers who latch onto a character. You write a misogynist character, for instance. Your reader gets creeped out. On screen, they might delight in a juicy negative character. On the page, it just makes the reader uneasy. Maybe they identify with a character who makes stupid or self-destructive decisions. They'll hate it. It colors everything they say.
There's the careless reader. These are not infrequently producer types. They probably read twenty pages. And they're giving you notes over the phone as they get frisked at the airport.
Guess what: listen to ALL OF THEM. But don't give any of them power over you. You don't have to agree, but you should listen.
I was preparing to market a script once. I gave it out to some colleagues to read through and give notes. One reader told me that the dialogue was very weak, and that I frankly should know better than to consider the work ready. I was more or less crushed. Next day a second reader responded. My dialogue was now "utterly phenomenal". I didn't agree with either reader, but I did realize that people have strong reactions to my dialogue, and it's worth revisiting that issue before I waste many a lonely hour trying to market the thing.
I went back to the script, and realized that I'd learned things since I put it to bed several months previous. I knew more. And part of the issue was the dialogue. But I ended up improving the script in several ways.
Another thing: your "real" reader -- the studios, Coppola, whoever fills that slot for you -- is not above these concerns. Hollywood employs legions of angry, frustrated English majors to weed through scripts. Some know their job. Others don't. Either way, it's their job to read twenty pages and skim the rest. Maybe they've read five shoot-em-up scripts already today, and the last thing they'll be entertained by is a misogynist and his girlfriend. Maybe your characters lack a satisfying emotional core, just like your barista keeps telling you. You don't have to please every reader, but there are always ways to satisfy readers along the way.