Valve makes some good games, I wont deny that. They are a little overhyped, though. I've never been impressed with the "physics puzzles" they've tossed into the HL2 series. The Episodes both have some areas you can see they wanted to stretch out the time played (EP1 had you run back and forth taking people to the train, EP2 had that antlion turret defense thing). Just things I can see people ripping on if it was anyone but Valve.
I don't consider myself a Valve fan. I've liked most of what I played from them, except TF2 and the original
Half-Life, which I actually tried to play for the first time
after HL2. To me Valve is just another company that makes games. As I said, I happen to like almost everything they've released, including both HL2 episodes.
Regarding HL2, I don't think I was as impressed with any other game ever save
Doom. It certainly wasn't the storytelling. I think it was sort of... personality the game had. The beginning of the game at the train station went a long way towards really making me feel like part of the game and I was impressed with the little things, such as the Combine guys pushing you when you got too close. And I liked the physics puzzles because I hadn't seen that level of physics in a game before. In short, the game truly floored me.
But I acknowledge that people such as Que had issues with the game because of how it felt like a bunch of different little games stitched together. For me that wasn't a problem and I actually liked it, but not everyone is me. I'm not a staunch HL2 defender or anything, but yes I personally think the game was fantastic... a milestone if you will.
The Source engine is a bit of a joke, at least to me. 2004 HL2 released, 2010 is looking like when Episode 3 will be out. 6 years, same engine. Updated here and there, but for the most part it wont be terribly different. Thats the length of time between HL1 and HL2s release, and they had to build the engine (or at least the parts that weren't from Carmack). We went from UT2004 to UT3 in this time, Doom 3 to Rage (should be out by then).
I don't think it's a huge deal. It's beginning to show its age, but I wouldn't say that games using the source engine look dated yet. Look at the longevity that the QIII engine had, and it definitely was looking dated after 6 years.
I think you do have to wonder though why it took Valve so long to come up with Source. It was an evolution, not a revolution. And it's not going to last forever no matter how many little updates they do. You have to wonder if they are going to take another seven years to make the next engine.