I loved this game. It brings Half Life back to the slow-paced exploration and puzzle solving of the original. The story doesn't progress much, has few ok bosses, no new weapons, no new bad-guys and no new environments, but the dialogue from Alex is great. The character of Alyx feels so much more human this-time around and her personality as an NPC really shines through, which although done well in many RPGs is a first for an FPS.
I am now going through the commentary just now and I spotted this:
It is an omission by the developers saying that during development they realised players didn't like Alex telling them to hurry up all the time. Which is exactly why I hated Half Life 2 so much. If it wasn't Alex or another NPC telling you to hurry up all the time. The buggy in a way even felt as if it was doing the same because that just raced you through load screen after load screen. It destroyed the wonderful slow pacing of the original.
I did think that after completing the game I was going think it was too short, but it is much longer that Blue Shift and possibly longer than (although it has been a while since I last played it) Opposing Forces so ye it's worth a purchase certainly. And so far the commentary has been good, although it doesn't pause the gameplay at all.
4/5
EDIT: There are two new types of enemies. A flying thing which you are introduced to, but won't be able to fight until episode two, a new type of Zombie called a Zombine, which isn't anything to get excited about and additionally you actually get to fight the Stalkers, which technically aren't new.