Mostly dreadfully bad
(If you're a stickler, there are spoilers following. But they're not critical to any plotline, like divulging that the butler did it or something.)
I'll admit, I popped the "Moderns" DVD after only half an hour, so maybe it got better. It couldn't have gotten worse.
I saw only two positives in the half hour of my life which I'll never get back.
1) The art direction/production design was very good and a fine representation of Paris in the '20s (not that I'd know from personal experience, mind you). But I've been, and done enough studying of French culture to have a feel for it.
2) It was mild fun trying to pick out the famous personages being portrayed, very much like looking at a Hirschfeld drawing (appropriately) of (his) contemporary notables.
I got as far as the salon scene where we first see Stein. When she starts ragging on people, I immediately wondered if she had been that much of a bitch with no one calling her on it. Then when Carradine slaps Fiorentino and Lone just stands there instead of laying him out flat, I gave up. Perhaps it was to ba a delayed "hit" of one kind or another later. But the dull acting, the blasé, detached responsiveness of everyone, whether true to the historical reality or just deadened acting, I hadn't the patience to find out more.
I do realize that it's a 1988 film of 1926 (?) times, so maybe it just hasn't aged well. Take my critique for what it's worth, just one man's opinion.