The game's a good experience, mayhap not living up to most of the hype it generated. Like any other good FPS, it has its charming moments but, in terms of revolutionizing the genre, it wasn't on the same level as its predecessor at its time. Then again, Half-Life happened to come at just the right opportune moment to wow people. Over-zealous fans accredit all the glories found in atypical FPS to Half-Life such as cinematic level sequences and clever AI (well, at the time) but I believe that all those would have come about regardless, though it may have taken more time.
In any case, I'd say HL2 gameplay delivers. It really strives to mix gaming experiences together; you'll shift from outright running and gunning to the judicious use of limited ammunition on hordes of zombies (ahem, *headcrab-challenged individuals*, to be correct) to frantic fast-paced driving. Good stuff.
Valve's implementing the grav-gun, in my opinion, was its saving grace from being dismissed with other FPS's. Flinging loose debris, explosive barrels, and razor blades at enemies has got to be one of the most gratifying gaming moments. And, the grav-gun was the developer's chance to show-off some of the game's physics which would have been overlooked without the grav-gun. I mean, we've all seen ragdoll corpses toppling over by now.
Still waiting for the multiplayer part of HL2 to deliver. CS:Source is just a new facade to an ultimately unchanged game. The snazzy new models and effects (...just wait until you eat a flashbang grenade head on) are nice but they don't really add that much depth. And the physics engine, implementing loose objects, can get annoying at times, though I laughed fit to burst when some players actually barricaded a pathway in the cs_office map with file cabinets. Still, this is a game most of us have played to DEATH and fancy new looks simply can't hold my interest for long. In any case, I'm more hopeful with the Day Of Defeat: Source release (whenever it comes).