Having recently completed Half-Life 2, I guess this is as good a time as any to offer up my assessment.
First, the good stuff:
It looks amazing. Even with an engine that's not as advanced as Doom 3's, it still looks better than Doom 3 does. It almost looks as good as FarCry, lacking only FarCry's incredible viewable distances. Particularly impressive is the water effects, stunning!
The other characters in the game are startlingly realistic and believable, there were times I almost forgot they were just some pixels and polygons. Top notch voice acting and an incredible amount of facial expressions made the characters, most notably Alyx, come alive.
The game play, while not groundbreaking, is still excellent. At it's core, it's still just another FPS, but the additions of vehicles and physics-based puzzles (among other things) help differentiate it from the pack.
The story line is intriguing and engaging. This is but the second part of a larger story, so don't expect any answers to many of the series' mysteries, but Valve does an excellent job of feeding you enough pieces of the puzzle to keep you interested.
They've continued the trick from the first game of never breaking the first-person perspective. It really helps in the immersion when you see everything through Gordon's eyes. It's especially nice during the cut scenes, such as they are. You maintain control of your character and can look around and move about while the story's exposition continues. I hate cut scenes, so this is big for me. I want to play a game, not watch a damn movie.
Now, the bad stuff:
It's buggy. Really, really buggy. After more than four years of development, I'd have expected better.
There's the stuttering issue, where every time the game auto-saves or needs to swap textures the game freezes up for several seconds and the audio starts stuttering over and over. Depending on your system specs, this might happen only during auto-saves, or it might happen every few seconds. I generally only have the problem when it auto-saves, so I guess I got lucky in that regard. Still, it kills the flow of the game on a regular basis.
The thumbnails it creates in the screens to load and save games are, more often than not, garbled nonsense. Certainly not a huge deal, but why have them if they don't work anyway?
A patch is forthcoming for these first two problems, but they keep promising it "tomorrow" and then delaying it another day, so we'll see how long it takes to actually be released.
The loading time between levels is much, much longer than in the first Half-Life. It's still shorter than most FPS games, but it's not nearly as transparent as in the first game. Quick saving is also not so quick this time around, it takes several seconds on average. Neither are deal breakers, but it hurts the immersive nature of the game when your taken out of the flow regularly.
As cool as the physics are, they're not without problems either. I occasionally found myself getting stuck on some of the physics items. It's easy enough to get unstuck, usually just buy ducking or jumping, but I took a pretty good beat-down on more than one occasion because I got stuck on a chair or something while attempting a strategic retreat.
Changing display settings take a really, really long time. It locks up my system for as long as several minutes, depending on what I'd changed. First time I did it, I thought the game had locked up completely and was reaching for the reset button when it finally finished. Not a huge problem, but it was really frustrating when I was first getting things set up and tweaking the graphics a bunch.
Beyond the bugs (and those are just the ones I encountered, check any of the Half-Life 2 forums and you'll find there's a lot more) there were a couple game play issues as well:
The hovercraft levels dragged on way too long. I was really, really bored with it by the time I finally got to ditch the damn thing. It was fun for a bit, but they overdid it.
Jumping puzzles. I hate jumping puzzles. It's not 1995 any more, let's move on from the jumping puzzles. They're stupid and annoying. They were a bit better than in the past, as none of them were the "miss the tricky jump and you die instantly" style, you could usually recover from a mistake and try again. They were also mercifully rare through most of the game, except for one section. That level ("Sandtraps") was essentially one, big, long jumping puzzle. Argh! At least it was forgiving about it, so you could recover easily from any missed jumps. Plus, it gave a great opportunity to play with the Gravity Gun!
The biggest annoyance in the game, however, was during the levels where you had team mates. If you were moving about through tight spaces, which was most of the time, they were always in the way. It made walking from room to room a complete pain in the ass, and tested my patience time and time again. Their habit of always being perfectly in your way was even deadly at times. On many, many occasions I'd be in the middle of some big fire-fight with some Combine soldiers and need to take cover, only there's a team mate standing there blocking the doorway, leaving me wide open to go down in hail of gunfire. Grr. The least they could have done was enabled friendly fire, that way every time new team mates joined I could just cap them and move on. Horrible thing to do, yeah. But, more often than not, I was better off without them.
So, what's my final verdict? In spite of the negatives I mentioned I think it's an amazing game. It could have been a little longer, but it was by no means short. I was just enjoying myself so much that I was disappointed when it ended! I'd rank it as one of the best games in the genre, and one of the best period. It's good. Really good.
Oh yeah, about the ending, I won't spoil it, but don't expect any resolution or finality and the end of the game. There's definitely going to be a Half-Life 3 from the looks of it. :-)