Wow, a month later, finally finished the main plot line. Holy crap this was a long game... but one of the best I've played. Definitely at or near the top in terms of storytelling and characters. I really loved the overall vibe of the game; intense missions when you want them, interspersed with lots of outdoor (well so to speak) activities at your leisure. Really worked for me. Terrific graphics too.
I didn't complete all the optional stuff like the hunting and fishing (though I did the majority of both of those), barely even started on the bones, rock paintings, card collections... there's just so much to do. Even now that the main plot is done, I could see myself returning to it once in a while just go do some of that stuff, to "hang out."
(I very much have in mind a scene from Big Bang Theory where Sheldon is playing RDR1, and just wandering around on his horse; he explains that his doctor or somebody suggested he "go for a walk.")