I have a brand new Mac, so I'm pretty sure it will run smoothly. I'm not a normal Steam user, a friend at school recommended it to me, so I've decided to try it out.
check the specs. just because you have a new computer doesn't mean it will run smoothly (though if it's a new mac, chances are it will)
if you're patient enough to wait, i would really suggest waiting for a sale. you'll be kicking yourself when the game goes on sale the weekend after you buy it, otherwise.
i don't know if they will put it on sale (since halloween is over and the xmas sale was the only reason it was cheap) but it might be worth waiting a week or two if you wanna save $10 or so.
it isn't a horror game, it's more of a horror sim i'd say. which is odd. maybe not a sim, but it isn't a traditional horror game in the sense of it shouldn't be treated as a game. you're supposed to immerse yourself in the game to get the best results (play at night/dark room/headphones/etc), otherwise you'll be disappointed. the most fun is not knowing what's around the corner, what's at the end of (or what will come from) the dark hallway.. and general atmosphere.
good stuff, i need to finish it. i've played about 3 hours i think. maybe 5. steam makes keeping track of it nice and simple, and has some neat-o community features. worth checking out.