I started enjoying this movie when....(spoilers)
First off, let me say that I'm 52 years old and have a reasonable knowledge of cinematic history.
When I first saw the trailer to this movie, it did nothing for me. It didn't look funny at all and had that director vanity look to it where they just grab as many of the most popular actors at the moment and throw them together to help replace actual storytelling.
Having said that, I watched Hail, Caesar for the first time yesterday. Going into it, I was aware of a lot of the negativity surrounding the film and was expecting it to be what I thought going in. And in the first 15 minutes of watching, nothing had dispelled my preconceptions. The "humor", I found to be weak at best. But once I stopped looking for laughs and started enjoying the acting, the sets, the homage to a bygone day of filmmaking, it got better for me.
The Coens did a wonderful job of casting and Josh Brolin, as others have said, was fantastic. Since he has the most screen time, he was a wonderful choice. I also enjoyed Tilda Swinton (great costumes), Ralph Fiennes and of course, Alden Ehrenreich. This is a visually attractive film and I found myself getting lost in the 50's with it's own unique style.
But where are the laughs? For me there really wasn't any. The scene with the thief cowboy obscuring the moon by diving into the horse trough says it all. There's nothing funny about it to me, yet the "audience" is howling with laughter. That 50's audience may well have reacted that way and that's my beef with this movie. The humor is of the kind my parents MAY enjoy, although I think they need a little bit more than that now. Others will see all of the so-called hidden meanings and references to whatever and think how clever it all is, but it's honestly too much work for me. Even with some knowledge of 50's cinema, it's just not funny.
So, in the end, I did enjoy Hail, Caesar but maybe not in the way as the Coens had intended.