Gives you something to do while waiting for the next Half-Life installment
I have to admit after finishing the game, I didn't really understand the story much, but it seemed interesting at least while it lasted. Time-travel and cool gadgets and some interesting characters are enough to get me to at least finish the game.
They do the multiple endings thing too, but at least it's a quick autosave 5 minutes reply to see them all.
The gameplay was pretty good, and the video and audio were also very impressive.
In summary, if you like a good single player game with a decent story, give this one a try. I can't believe I didn't really hear about this game before seeing on sale.