Yeah, I never take leaked alphas to be a full indication of what the game will be like. It gives you a small taste, and shows you a small glimps of what to expect, but you shouldn't judge a game on it. I really liked the HL2 alpha, because as a map designer/3d animator myself I really got a chance to see the game in the form it's currently being worked on, rather then only seeing the end result. It's like looking at a painting before it's finished. And especially after seeing some of the problems they have to overcome, I can better appreciate how much work they put into the game.
But in the same way, even seeing a few suprising or impressive things in a game's early alpha gets me excited for the game. When the Doom 3 alpha was leaked, what did we see? Basicly the exact same thing that we saw at E3, except that we had more time to look at it. Even the Half-life 2 build showed things we had already seen at E3, except now we can play it for ourselves. Sure, it gives us access to things we aren't supposed to see yet, but really most leaks are just extended versions of content we've already seen. But then again they also reveal that what we see is not always what we get (like the soldier kicking down the door in HL2 being scripted...)
EDIT: Okay, so maybe it wasn't real quick
Yeah, my posts are like that too