It's awesome. Definitely worth it. Buy it through steam ( www.steampowered.com ), don't bother with retail.
The game is very very cool.
I'm thinking of getting it. the mid-level package from Steam sounded good. you get 2 other games... but like yay, I kinda stopped in the middle of HL1, can't remember why now. I like the aliens idea though. much better than demo*s on mars.