If you haven't bought the originam RDR, then why not?
You don't really need to know the original characters to play this excellent expansion, but it helps. For example when Marston meets up with Bonnie, many will not understand her forlorn look when he hits the road again.
Onto the game, it's a fairly basic shooter when all said and done, with looting the erm..dead undead an absolute essential passtime, as of course there are no stores with which to buy anything.
Saving the towns is fun and Rockstar have made it so that you can continue the storyline with a decent lapse of time before they come under attack again, so you're not continually having to redo something you did twenty minutes beforehand, even though the whole system is the same for each town, which some may find a little repetetive.
Ammo are the dollars of the original RDR so don't leave any corpse unturned (hoho!) and from time to time you may come up with a goodie other than bullets.
The back drops and scenery are superb and a real feeling that the apocolypse has come to New Austin dwells throughout.
What makes this game for me though is the music score. The incidental music is eerily grating and dignified of a suspenseful horror flic with that "scraching a blackboard with your fingernails" topping the bill.
The discovery of the four horses of the Apocalypse is also noteworthy as is the undead blunderbuss.
All in all a great addition to the RDR collection, which still leaves us with endless offline options in the future.