YOUR OPINION DOESN'T MATTER
Wow! What kinda jerk says that?
Well, Broadway composer Stephen Sondheim does. And I agree (to an extent).
The article found here tells the story of some criticism Sondheim received when opening a new show. I'm summarizing here, but essentially, his point is - there is a time for criticism, and there is a time to show full support.
Whenever you go to a film premiere or first see a completed work of some kind, you should tell the artist that it's wonderful (especially if you're a friend). Even if the work is horrid, you should say "Fantastic job!" in the most enthusiastic voice possible. The reasons for this (in my experience) are as follows:
1) This person just accomplished something, and likely it was something major. They should be allowed to celebrate! Help them to do so.
2) The work might not be to your tastes, but who's to say it was made for you? Maybe the artist made it for themselves and loves it! Good for them!
3) If the work is complete (especially in the case of film), the artist can't go back and change things. Giving them your criticisms at that this point doesn't help anyone. (But save those thoughts to share before they start their next project, but only IF your friend asks for them.)
4) Likely, the artist knows what the problems are with their film/play/etc. They were in the audience and heard the same reactions you did. They know the weakness of their work. And if they can't see it? They will in time. Trust me.
Once, I came across a filmmaking friend I hadn't seen in a couple of years. He had written a feature script and was a couple of weeks away from shooting it. He was so excited and wanted to get my thoughts on his script. I took it home and read it that night.
It wasn't good.
The next day, I talked to the friend, told him the script was terrific, and offered to help out on set if he needed it. He thanked me a bunch (but didn't end up calling on me).
Why did I lie? Because of where he was in the process. He was about to jump out of a plane, and I didn't want to tell him his parachute was looking iffy. I knew he needed to believe in his project at that particular time more than ever. Plus, as mentioned above, maybe he was going for something that I just couldn't envision.
Eventually, I went to the premiere for this film. All the problems I saw in the script were there on the screen. When I saw him after the screening, I could tell he saw those problems too (even if he didn't fully understand them yet). I offered him an enthusiastic congratulations and a hug, because that's the only response he needed. Always spread the love!