There's a million reviews for this game if you haven't already heard of it. A slightly different take for me - I bought this game fresh with my Xbox One X and I haven't played consoles for about a decade because I have young kids, so it's the first game I had played in a good while.
I had not played RDR 1, so I came in fresh. You don't need the backstory, but also I expect it does make for a different experience if you have played the first game (I played RDR 1 afterwards). Cons are that the game does assume a little previous knowledge early in the game, but Pro is the ending is much less predictable if you don't know the original story (surely by now you know RDR2 is a prequal).
Don't be discouraged if you have no idea who anyone is or what you are doing in the first few missions; as soon as you come down from the mountains with the gang, everything fleshes out and I'd consider this the true beginning of the game.
I gave the game perhaps 100-120 hours total, not exactly sure because it doesn't give you a count like RDR 1 does. I played most (if not all) side missions and did a bunch of other challenges like hunting, fishing, dinosaur bones, rock carvings. Try to get a list of where these are from the internet, mark them on your in-game map and do them on your travels, so you aren't laboriously backtracking across the map at the end of the game. I didn't do all the legendary animals, because once I finished the missions I felt I'd also done enough hunting.
Graphics are amazing, blew me away with what consoles are now capable of. To think I thought XBOX 360-era games looked good! Do yourself a favour, early in the game, find the nearest mountain top, climb up and look around. Amazing. Even at the end of the game, if I found myself on a bridge or mountain top chasing some objective, I'd stop to take in the scenery.
I developed a real attachment to the characters and Arthur Morgan himself is great - conflicted, genuine, both simple and complicated. Voice acting great, character sync and facial expressions about as good as games get (of course not flawless). Horse bonding is terrific, so many folks emotionally invested in their e-horse after 50+ hours of riding around together.
Game constantly surprises with what it remembers and how people behave without obvious and choreographed scripting. I've seen reviews that some folks felt it was a bit like puppeteering, but I just don't know what they expect from a 2019 computer program.
End-game decisions by your gang members are not entirely consistent, but consider this a bit like a Marvel comic - it's for entertainment and dramatic effect, rather than real-world realism. Similarly the game forces you to do some unsavoury stuff even if you are trying to be a pious Arthur, because after all, you are supposed to be some kind of outlaw.
The game can also be extremely funny, both intentionally (YNNEL!) , or just with the random stuff that happens on your travels. Tip - investigate everything! You come across some crazy and ingenious stuff when you explore.
Amazing experience, I'll prob play it again in a year or so.