The release was a pain in the ass, but I got it working, and I've been playing Half-Life 2 for the past three hours. My thoughts?
FUCKING. AMAZING.
This game is the very reason I advocate PC gaming over console. The graphics are jaw-droppingly advanced (literally), and the gameplay feels like you're in the center of a well-directed action movie. This game is the future occurring right now; Half-Life 2 will set the standard for the hybridization of movies and video games. The gameplay is perfect, the sound is terrific, and the enemy AI is intelligent as hell (during the scripted scenes, anyway). The ambiance is what really sets this game apart, though, as Valve has crafted a beautifully textured and nuanced world. While you need to play it to get an idea what I'm talking about, this game is top to bottom the best PC game ever, a surefire lock for game of the year, and an instant classic. GET THIS GAME.*
Caveat: This game requires some serious computer to run at the highest settings. I've got a P4 3.2, a gig of Corsair XMS RAM, and a Radeon X800 Pro, and I still get a little slowdown at 1024x768 with all the settings set to high and 4x anti-aliasing on. Almost makes me wish I'd picked up a X800 XT. Almost. Only invest in this game if you're running a cutting edge machine, or close to it.