I thought Ep1 was so-so. You're definitely right in which it was too short, plus on replay, the whole underground with zombies is a level I would gladly pass up, every time.
Ep2 was pretty good in regards to advancing the story. The Strider fight at the end was a little bit different, but felt fresh because you couldn't kill them by normal means, and the antlion guard chase got my blood pumping pretty good.
I'm wondering what the big hold up for Ep3 is. Either they didn't have a set storyline before HL2, and are stuck in writer's block, or they're building a new engine for the sake of keeping up with technological gains. The longer it keeps getting, the more I believe they just decided to move on to HL3, with a brand new engine and everything.