I enjoyed GTA4 for the first week or two I had it, and then got really frustrated with the mission structure and lack of saving, and was annoyed by the terrible shooting mechanics. I've probably already put more time into this than I did GTA4, and I'm still incredibly hooked. The graphics / physics engine, and the format of how you acquire the "story missions" is the same structurally as at least GTA4 was, but most of the complaints about that game have been fixed or refined, and the general setting and overall game play ends up feeling quite different because of the period. For big "open world" type game, this is probably the best there is (and you know how much I loved / worshiped Fallout 3), and the most "real" feeling world as well. It's also far and away the best "western" game there's been, and the uniqueness of that setting really pays off in making it fun and interesting to play. This is one that you could possibly rent for a week to decide if you like it enough to buy. It's absolutely enormous game, and there's tons to do outside of just the main quest type things. I've seriously put a major amount of time into it already, and I'm only like 20% through the single player. It seems like the type of game to me that most people could find at least something they love in it.
Plus, you could ride in a posse with NSW and I over Xbox Live, and commit old timey hate crimes against both man and nature.