Finished this earlier today. Stellar game. It's not the best thing I've ever played and it's not even close to being my favorite type of game. Specifically, anyhow. Yet, it impressed me. Hell, it amazed me.
The biggest downside to it that I could glean is that it's limited by what it sets out to do. Namely, puzzle solving. But whereas the original was short and with a relatively uniform setting or environment, this sequel certainly has some variety, and a lot of length, to it.
Personally, a must-play. Alongside Half-Life 2, Warcraft 3, Fallout 2 and... not many others.