A lot has been said about this game already, but I wanted to give my own opinion. And what a better place than this. Overall, the game deserves a good score. If you liked HL2, you will certainly enjoy this one because it is more of the same old and well-known things. At first, it feels a bit slow because there are lots of dialogues, not to mention that you go unarmed (except for the gravity gun). This is because the game starts exactly where HL2 ended. Afterward, you get your weapons again and it becomes as active as you'd expect.
If you are wondering why I am posting this now, it is I just finished the game today. If I have to say something negative about it is that EP1 feels too short. Its play time is estimated to be around five hours; I surely spent some more, but it still ended too quickly. HL2 was a bit more than twice the price, but it was much, much longer. If EP2 and EP3 are as "short" as this one, they'd benefit from a cheaper price.
And until EP2 comes out (will be a long wait...), I will go through EP1 again with the developers' comments enabled to see what I missed :-) Pity my machine is not powerful enough to enable HDR and other quality effects in Source.