I just bought the OST off Itunes/Amazon this morning and I gotta say, it might be even better than the original theme.
My top 2 favs from the original are Home and The Last of Us, though I kinda like most of them.
The OST of Left Behind adds all the ones that were missing in the original [main menu music, etc] and then some, My 2 favs of the DLC were Fleeting and Left Behind... Thees are just great but I liked most of them again [maybe overall more than the original OST]
The thing is that when I initially bought the OST prior to playing the game, I thought it was nothing special... Now that I've played it many times and the DLC, I find the scores amazing [though my music preferences have shifted somewhat so... that might have some effect too].
I now listen to music through open headphones almost exclusively and these OSTs are really fabulous through headphones [like the whole game... I think it's almost like the game and DLC were made for cans!]
Gustavo Santaolalla did an amazing job on both and I really encourage you to buy this one and if you haven't yet the original as well.
The memories it brings... You almost always connect a tune with a particular scene from the game.
Amazing, simply amazing.
A perfect score fitting a perfect game :]