I love the Game Fly service but this game has a save bug that makes it so that I had to start over. Until the game publishers come out with a patch, I would suggest holding off unless you cant wait and want multiple save files.
The short: If you played it on PS3/Xbox360, you're not getting anything new here. It is the same game but made better only for those looking for something to show off their PS4/Xbox One's capabilities (if so, also see NBA 2K14/FIFA 14.)
The long version: Tomb Raider was without a doubt one of the best games last year. It was fun to play, responsive, had an interesting story, and was all around a great looking game. Fast forward a year (or so), and you've got what is truly the 'definitive' (sorry) version of the game. It looks absolutely stunning and is our first look at what next-gen consoles can produce (it will only get better: compare launch titles of past consoles to the later ones...) The campaign length varies depending on what you're wanting out of the game: roughly 8 hours for the story alone and up to 16 hours or so to gather all of the collectibles and fully explore the island - tombs and all. However, there is a fast travel system implemented, so backtracking isn't nearly as painful as it could have been. There is also multiplayer, but I've honestly not spent a great deal of time with it. What I've seen and read led me to believe it was somewhat tacked on, but not by any means terrible. This is a game worthy of your time if you've not played it, and heck, even if you have, it's worth going back through.