Brilliant game... I got it on Monday, had a public holiday on Tuesday and played it all day yesterday.
There are three Acts with I think 8 - 10 levels in each and each level takes about 30 minutes to finish.
I LOVE this game... it's one of the most immersive games i've ever played... i'll definitely play through again on a higher difficulty.
I haven't finished it yet, but god damn the story is fantastic, the gameplay is fantastic, the weapons all feel good to shoot...
I will play this a lot, unfortunately there is no one on the multplayer servers yet so I can't play multiplayer all that much.