Heh, actually, I totally agree about the plot veering in a crazy direction. "Good job beating the alien invasion! Between games NEW aliens invaded while you were... ~mysteriously absent~. They look very much like aliens from one of the popular expansions, but let's just declare that noncanon and be super mysterious and never explain everything."
They really designed the new plot, and gameplay, around the engine. Which is a good engine, I love Source games.
But even the episodes were basically "Check out these cool additions to our engine! Also, have some more unanswered questions and a shocker ending!" and "Woah our engine can do these cool portals! Uh, sure okay it's the same setting as Half Life, since you're clamoring for that."
That said... The gameplay was very solid and aged well. And the story, while laughably unrelated to Half Life 1, is pretty decent for an FPS It relies waaaay too much on unexplained mysteries... Particularly when they were promising the episodes would come out quicker than full games. The "Half Life 3 confirmed" meme is just free publicity for the stuff they actually care about, a couple of overgrown HL1 mods (Counterstrike and Team Fortress). And fricken DOTA, haha. And Portal, yay!
No but seriously the gameplay is pretty good, and also the developer commentary is an *amazing* feature. They interactively explained so many neat things about the game design process. I give them a lot of credit for that, even if it was more like a documentary of game design than an actual mechanic.