Well, "the ship common.gcf" is 1.3GB. Like HL2, that hold the majority of the content. Then you have a couple more depending on which variant you are playing:
Multiplayer =230MB
Singleplayer = 375MB
Tutorial = 260MB
Though I believe the guest pass only allows multi and the tutorial.
So from what I've played so far its pretty fun. I haven't played multi, just checked out the tutorial and singleplayer. The SP so far is really interesting. Starts out with a little scene of Mr. X explaining that the ship is "lost" and the only way off is to play his game...which is to kill the other passengers. Your first quarry is right outside your room sleeping and easily taken care of. Then a bellboy talks to you and says he knows whos trying to kill you. He comes up with a plan to drug a guy that looks similar to me so he sleeps, then the bellboy will tell my potential killer that its actually me so she'll kill the wrong guy.
I go down to the infirmary to find the sleeping drug (syringe), locate the other guy and have to poke him while no one is looking and none of the cameras are facing me. Done. Follow him till he falls asleep, and alert the bellboy. Mission done.
The ship itself is one large map with lots of rooms, people, objects to interact with, etc...no loading in between. So when I'm done with the bellboy he sends me to another guy who gives me some missions, and from there another. Its all really cool and seamless.
While you're doing this you have your own needs...and quite a few. Gotta sleep, bathe, eat, drink, piss, poop (yes, they are separate...but you can do them at the same time..heh), read...and I'm not sure if thats all. Anyway, you need to deal with these things while you're completing missions around the ship. Stop in and have a bite to eat, take a snooze (in a bed or chair), go to the bathroom (wash your hands after to lower the bathe meter). There are items that can help you deal with these tasks, like carry a bag of chips and a can of soda around so you dont have to go back to the dining room to eat and drink.
The things you do can have an effect on more than one meter. Drinking lowers your thirst, but raises you need to pee. Reading lowers your read meter and can wake you up or make you tired depending on what you're reading. Thinks like that.
It sounds kinda hokey, but it ends up working pretty well. Its more interesting than just running around killing enemies, you have your own needs to attend to. Plus the meters fill fast enough to keep and eye on them, but slow enough to not get in your way all the time.
So if you get to a Circuit City and see ir there...$7....totally worth it. Plus its multi and singleplayer, and you'll spend way more to get either one off Steam direct.