--SPOILER ALERT - THERE ARE SOME MINOR 'SPOILERS' BELOW, READ AT YOUR OWN RISK-
Well, I just finished my 2nd 'run through' (In that I was mostly listening to commentary, noclipping around and messing things up the 2nd time). I'm very impressed.
It wasn't any shorter than I expected (they did warn us), and I'm so glad they didn't simply make it a tech-demo borefest, instead choosing to showcase the tech's practical, in-game uses.
The cliff climbing was very well done, and the combine solider rapelling seems to be different to the CP rapelling? it looked much cooler to me. Nice little hidden bits and pieces, and some very cool details.
Difficulty was -perfect-, challenging, but not -hard-, just the way I like it.
The final shoot out with the chopper was nice, especially having it careen into the platform when it dies. Funnily enough, the first time I did that, I must've been standing in the wrong place, because when the chopper hit the platform, it broke up and fell out from underneath me, sending me plunging to my death on the rock below. Which was cool.
The new effects themselves are about what I expected (haven't played DOD yet, but after seeing all the screenies and videos, we all pretty much knew what was coming.) Pretty nice tbh, you can really teel the difference if you turn HDR off then turn it back on.
The commentary ws good in parts (esp the stuff about gameplay from the Australian and the setting/architecture stuff from Viktor whateverhisnameis), but some of the time the people sounded pretty bored. A little enthusiasm would've been nice from some of the guys wouldn't have gone awry, but then again, I guess that's why they're making games and not doing TV voiceovers

.
Just a couple of gameplay notes/things to try. First of all grab the Fishermans's spear with the grav gun and chuck it at some combine. It works brilliantly, and is a nice touch. Don't, on the other hand, try to kill a combine soldier with a rock. It takes about 5 shots to take 'em down, which is a little disappointing... I really thought that would work. In the church courtyard, try firing the Rifle's secondary grenade at the scaffolding with the ammo on it... be prepared for about 50 little wooden gibs at once. Very cool. Also, I really enjoyed knocking a bunch of stuff off the edge of the end platform, just to watch it fall the hundreds of metres to the ground below. Finally I haven't tried putting everything in the Headcrab gun yet, but it seems that it's quite easy to jam. first time, I jammed a bucket right in there and it caught on the way up. Second time I just dropped an ammo crate into the bottom of the shaft. Both worked equally well.
Anyway, that's my bit. Enjoy yourselves.