It's tough to know where to start when talking about a game like Red Dead Redemption, so I'll start simple and straighforward. You should play this game.
If you love westerns, you will love this game. If you enjoy a well-constructed video game with tons of stuff to do, you'll enjoy this game.
You'll spot elements of GTA in the mix, of course, but RDR goes above and beyond to offer a different game. It's not just GTA with horses, as some people are so quick to assume. It takes things that GTA did right, and uses them... and it takes some annoying factors from GTA and improves them (mission checkpoints, hell yes).
I stayed on the story path early in the game, because it functions as a tutorial, slowly doling out new abilities and activities at a pace that is easy to digest (although can also seem a little sluggish). You also start with no money, so I wanted a little of that in my pocket before I struck out. About five minutes after I set off the beaten trail to admire the scenery, I heard a guy shouting. I looked around and he was running up to me, asking for a ride because he'd been stranded. I slowed my horse, thinking to give the guy a hand, at which point he threw me off my horse into the dirt, and started to make off with my steed. He was fast, he got pretty far. He couldn't outrun my bullet though.
Yep, this is the wild west.
They say the main storyline will take around twenty hours to complete. Thus far I've spent at least and hour each just playing horseshoes and texas hold'em in game.
In fact, I ran out of people to play poker with. I purchased a suit that allows you to try and cheat at poker. If you get caught, someone at the table calls you out and challenges you to a duel. I got caught a lot, and I had to murder everyone at the table.
Throughout the game, there are over a dozen different outfits you can collect by completing various challenges, which give you different abilities and bonuses.
John Marston is a fun protagonist to guide through the game. He's quiet and a little dark, but a nice change of pace from the grizzled and gruff action heroes we get a lot of in video games lately. It's nice to listen to a protagonist that doesn't sound like Christian Bale's Batman with throat cancer.
When you take the game online, you're presented with what is basically an exact duplicate of the single player game world, in all its enormity. This is your game lobby, and you can easily just spend all of our time here, running around with your buddies, shooting eachother or teaming up to kill bandits, or... shooting eachother. You'll be shooting eachother a lot, it's terribly hard to resist.
You can also jump into constructed games like deathmatch or goldrush. Goldrush is the real gem here. Bags of gold spawn all over the map, and it's a mad rush to scoop them up and return them to crates to score. You can carry two bags, but it will slow you down. It's frantic and violent and some of the most multiplayer fun you'll have.
RDR multiplayer is not what I would consider incredibly balanced and competitive. It's not going to be the next MW2, you probably won't see clans and leagues running around. But it is incredibly enjoyable in its chaos, and on top of however many hours the single player game will devour, you can bet you could come close to matching that in multiplayer.
I was a little disappointed to find out that Texas Hold'Em and horseshoes were not available in multiplayer, but I'll just keep my fingers crossed for DLC.
RDR isn't perfect, but the issues I've come across are so very minor in the face of all it does right, they won't do anything to hamper your enjoyment of the game.