Don't get the four stars wrong - this game is among the best there have ever been. I'll explain the missing star as we go on.
The single player campaign is pretty awesome. I've played for quite a few hours (15-18) and have completed about a third. As with GTA (IV, mainly - and, yes, the comparison is obvious) you can choose to follow the main story or just go about your day any way you feel like. I've been doin' both.
A pretty useful tutorial is included in the first shaky steps of your heroes tale, and make you feel comfortable with the way you move and the things you could do. If you've played 3rd-persons before, you'll get a hang of it right away. It presents a couple of the main NPCs and the cutscenes tells you alot about John; the man of the hour.
As the story unfolds it gets you to move around the impressive-sized map, and presents the many different things you could do (if you don't want to do just mission after mission) like hangin' in a saloon - playin' poker, Black Jack, dices or havin' a drink - or shooting different kinds of animals (collect the skins and sell them) or explore the map and meet all kinds of different people with mini-tasks for you.
As with the Fable-series and Fallout, for instance, the hero's progress and actions can be reviewed by checking the Honor- and Fame-meters. Honor is gained or lost by doing good or bad things (and people react different to you depending on the way you behave). The fame-meter is a result of what you do in a more neutral way - everything you do can make you famous; saving a prostitute from a drunk bad guy will give you fame-raise and honor-raise - gunning down an entire village gives you fame-points but decreases you honor.
There's not loads and loads of different guns to be used, maybe around 30, but it's good enough. When added with the other things you could buy or earn, the inventory is acceptable.
The gameplay is good. It's not perfect. In the beginning it could be a little tricky to control your horse while you try to shoot your rifle - but it actually just make the game more enjoyable. Some people have been complaining about the Dead Eye - and my answer to that is as simple as it is obvious: if you don't like it, or thinks it makes everything to easy, don't use it!
All in all you will enjoy the single player-campaign. For me, I will play it at least twice - one as a good guy, and certainly one as a bad guy. That makes the game last for quite a while...
As many else have written, the multiplayer starts in the free roam-mode. You can roam the whole map freely, forming a posse and fighting either other players or the NPCs roaming around. Maybe go for a hunt or just some sightseeing?
If you don't get a kick from that, try the different kinds of multiplayer-matches. The classics are here: capture the bag (flag) and deathmath for instance. One funny thing about the mp-matches are that some of them start of with a typical wild west-shootout. Hey, Wyatt and Doc Holiday... feels like you're lined up behind the O.K Corral! Really funny.
But... I feel like a want something more. Yeah, the graphics are really good. Yeah, the single player is really good. Yeah, the multiplayer is really good. But... it could be grand, fantastic, totally awesome - EPIC! But it's not. With just a little more effort the game would be the best one ever created. Example: Why can't Rockstar allow us to create our own heroes? Why can't I decide to shack up with my family and become a farmer - only to sell my crop in the local village? Why can't I become a dreaded outlaw with an army of lowlife-comboys under my command?
It's so close to the ultimate action-strategic rpg... but it's not.