I've been through the game twice (easy and normal). This game is the bumdiggety and a worthwhile sequel to Half-Life (which I've played countless times). The grav gun (and it's cousin - sssh - don't want to spoil it) is the most ingenious "weapon" ever devised for a first-person shooter.
I got a free copy with my ATI card and downloaded it in chunks over the past few months before release over my cable modem. My experience with Steam was completely seamless and trouble free. I have yet to burn a backup copy of my game to CD, but I hear it's possible. I don't have a problem with the required internet connection or any of that. The whole thing has gone quite smooth and my PC is online all the time when it's on anyway.
The game runs pretty smooth on my machine: AthlonXP2000 (nominally a 1.6ghz cpu), half-gig RAM, ATI Radeon 9600XT 128MB. I have all settings on medium, models detail on high and 800x600 resolution. I could probably get away with 1024x768 resolution, but I've got no complaints. I wouldn't mind a faster processor - levels take a while to load, and a 256mb video card might be nice.
Plot-wise I wish they would explain just a wee bit more. But there's a good
story summary available. Be warned - the game may leave you with more questions than when you started.
Additionally, there's a lot of good info on the Steam forums. A lot of people are bitching about sound stuttering problems and such, but I've had a great experience playing the game. It's very intense from the word go and it immersed me in the story more than any game since Deus Ex.