I've been a gamer for a long time and yet somehow this game still makes it into my top 10 favorite games of all time. The environment is rich and engaging, you'll find yourself spending a lot of time doing "nothing" - little side missions, hunting, finding the best location to watch the sunrise, etc - and just being too entertained to stop. If you get this game and enjoy it, I also highly recommend the Undead Nightmare add-on pack. It basically gives you an entire new campaign to play, except in a zombie setting. One of the best game add-ons I've come across.
The gameplay is spot-on, probably because Rockstar has mastered these types of 3rd person games. The controls work almost flawlessly, and the horse-riding is one of the most intuitive experiences I've had in a video game. Most games that introduce horses don't focus that much on the mechanics involved to have a character ride a horse, but since this is a western game this is a very detailed aspect.
There are many adult themes, so it's probably not one for the kiddies, but for any even semi-mature gamers, this one is definitely one you shouldn't miss.
Really, I can't say enough good things about this game.