A Chapter By Chapter review.
Point Insertion/Red Letter Day: 7/10. Nothing really exciting here, but exposition is always important, as is getting familiar with the controls.
Route Kanal: 6/10. Easily my least favorite part of the game. It's just a bunch of tedious "puzzles" to me.
Water Hazard: 6.75/10. Slightly better than Route Kanal, in that you get to shoot that goddamn helicopter out of the sky.
Black Mesa East: 8/10. Short, but sweet. N Ice
to not have everything shooting at you for a few minutes, and the getting the Gravity Gun is a plus.
"We Don't Go to Ravenholm..." 8/10. Infamous as it is, I give it an 8 simply for scaring the hell out of me at 4:00 A.M., and for having Father Grigori.
Highway 17: 6/10. The Antlions. That is all you need to know about this chapter.
Sandtraps: 8/10. I liked it, honestly. The "don't touch the sand" bit was hard, but it was a fun challenge, honestly. But still, poor Lazslo...
Nova Prospekt/Entanglement: 7.5/10. It was nice finally being able to order the antlions to maul the Combine soldiers, but I give this section a 7.5 because of those damn "set up the turrets and fight a bunch of soldiers while Alyx dicks around for a few minutes" parts in Entanglement.
Anticitizen One: 9.5/10. This is where I believe all of the truly awesome stuff starts.
Seeing the rebellion standing up and fighting for their freddom just felt great, and commanding squads of rebels was pleasant. The only reason it doesn't get a 10 is because of the end. It took me 3 HOURS to get out of that area.
Follow Freeman!: 9/10. Continuing from the last chapter, leading the rebellion was cool, and I liked the challenge of fighting the Striders. Also, I loved fighting alongside Barney, and I don't think he could have delivired a better ending line: "And if you see Dr. Breen, tell 'im I said 'F[crash] you!'"
Our Benefactors/Dark Energy: 10/10, Easily the best parts of the game. Losing all of your weapons except for the Gravity Gun is initially a letdown, but once you discover what it can do now that you're in the Citadel...it's just the most fun I think I've ever had in a single-player FPS.
All in all, though it may be a bit overhyped, it's definitely worth a weekend of your time, and any subsequent playthroughs.