Rockstar has put a lot of work into the details of the world but honestly most side activites just aren't any fun. Its good enough to play through the story missions once but not much else.
Also I didn't really like the main and side characters, pretty unsympathetic bunch.
90% of this game is riding from camp to your destination then you get 10 minutes of action and it's back to riding again. Also pretty much all the activities center around getting money but you don't really have anything to spend this money on. Once you have upgraded your favourite weapons and the camp your money just keeps piling up with nothing to spend it on. The whole story is about making more money that you can't use.
Don't get me wrong it was good enough that I actually played all the way through it which I don't do with many games but the hype is really overblown.