I've rented it, and I like the game. There are some things that I miss from the original, like the challenges, and the specialized bots. Not quite sure about unlockables other than you unlock new difficulties.
I've only gotten to about the 4th level, but overall the single player has the original feel. Co-op is really thought out as well, with levels where your partner starts at the street level, while you give sniper support from the roof tops.
Its really one of those games that you either love it, or hate it.