This seems to be a bit of a no-brainer (pardon the pun) as it works out cheaper (after Quidco) than buying the DLC bundle pack from the PSN store, has resale value, and is playable without owning the main game.
You are basically getting a six hour or so single player campaign, multiplayer/co-op zombie modes, plus all the other multiplayer and free roam modes that have been released. Everything but the original single player campaign, as far as I can tell.
"Rockstar has confirmed that Undead Nightmare will be sold as a standalone offering at retail. It bundles the Undead Nightmare Pack, the Outlaws to the End Co-Op Mission Pack, the Legends and Killers Pack, the Liars and Cheats Pack and all the Multiplayer Free Roam modes released to date — all on a single disc. Unlike the single-piece DLC, a copy of Red Dead Redemption is not required to play."
You will need to use a Walkers voucher to get this price. Quidco tracked at 57p for me, taking the price down to £15.58