It's a very slow-burn film with a muted colour palette and large swaths of dialogue-free scenes. It's also a film you need to focus on properly. If you're not in the mood/not prepared, then you'll likely find this a highly dull and uninteresting film. I can understand why some might not find it appealing.
That said, I went in feeling pretty tired, which I feel may have dampened things a little but that didn't stop the film keeping me engaged (which I suppose isn't surprising considering how 'The Assassination of Jesse James' is seen as slow by some but I find it fascinating). The same can be said of those audience members who liked making themselves known, whether it is by scrunching and tapping a plastic bottle or just announcing your thoughts relatively loudly over very quiet scenes. They ruin the viewing experience, but the film was strong enough to overcome that.
If you can get into the film, then you'll be rewarded with an intriguing (and dark) drama that expertly handles the relationships between the main characters (especially the brothers), each drawing very strong performances from the actors (and whatever make-up/prosthetics they've been given). It's not hard to see why Carell and Ruffalo are getting the attention. Carell's portrayal of the lonely and creepy du Pont is against type, so it's no surprise it's getting attention. What's a little disappointing is that I haven't heard too much giving enough credit to Tatum, who seems to be playing a typical role (he does unintelligent well, doesn't he? Just watch '21 Jump Street') but this is a far deeper role than you might expect and he pulls it off well. It's hard not to feel sympathetic towards him.
The performances aren't the only element that make the film work. It's a very well-made film with the slow pacing allowing the characters and relationship room to breathe (though it does also require a little patience) while the quietness and colouring amplifies the focus on the characters and brings out the isolation and emptyness surrounding them (especially du Pont). It's effective.
This is yet another biopic but it is one of those ones where you get drawn into the story that you forget it's a biopic. It's not one of those lazy, generic ones that just skims through details built around a performance. This one goes deeper than that, telling a compelling story with some interesting themes and character relationships.