Played through it and beat it in about 3.5 hours or so. It's about 1/7 as long as the full HL2 game, and it doesn't give you anything "new" to do. No new vehicles, weapons, or anything like that.
But, its still worth it if youre an HL2 fan. I had to have it the first day it came out because HL2 single player is probably the best single player FPS of all time (besides deus ex) and there was no way i was going to miss it.
It continues the story along a little bit, but not by too much. Without giving away anything - You go back into the citadel and try to prevent the core from destroying itself and causing a nuke that will blow up the city. After that the whole expansion is basically escaping from City 17 before it blows. Its pretty fun overall.. but like I said its short. It says 4-6 hours on the box but thats times for amateurs. I blew through that shit on hard difficulty while still taking everything in and admiring the graphics and the world they created
You can download it if you dont want to spend 20 bucks