It really comes down to what you want out of a story: do you want excellent story over gameplay? If yes, then the game is a must buy. If gameplay is supremely important, then I tentatively recommend you pass on this. The main focus is on building atmosphere, characters, and themes that truly distinguish it as a title. In this game nothing quite compares to just going for a horse ride in terms of sheer western atmosphere and beauty (when you first enter Mexico and the soundtrack kicks in is currently in my top 5 gaming moments). There is a fast travel system of sorts, but using it often deprives you of some of the most beautiful sights in the game and some interesting discussions on subjects including but not limited to, the role of civilization, industrialization, religion, punishment, and lust for power. And the ending of the game, is without a doubt my favorite ending in gaming. But I can't lie, game starts off at a snails pace and slows way down right before the amazing ending. If you can get past some truly dull tutorial missions in the opening hours of the game, you will find yourself hooked.
But whereas the game is truly amazing in the writing department, the gameplay is often sticky, repetitive, and often broken. The gameplay is ripped right out of GTA and when I mean ripped, I mean torn out violently. The gun play is just shit and relies entirely on a lock on mechanic. Missions outside of cutscenes aren't very varied or memorable. The sidequests are a mixed bag of interesting, if somewhat repetitive hunting missions, excellent stranger missions, and picking flowers. Yes, picking flowers. And it is as dull as it sounds. But there is something about the gambling mini-games that leaves me satisfied, save a few shit options (fuck horseshoes btw).
Overall, I would buy it, but then I already have it. My recommendation is to look deeper into it and find a cheaper version, as I have seen RDR for around that same price in quite a few stores. Also look into getting the edition that comes with the zombie expansion. Why does a western game have zombies in an expansion pack? Because nothing says I am a badass cowboy like pulling out a revolver and mowing down the zombie hordes while riding death incarnate. Or alternatively a unicorn.