Some time ago, I posted about why I don't use Steam to buy games. I still stand by what I wrote there, but I have since installed Steam for one reason: Portal was free, and I've been keen to play it for ages, but never had such a good opportunity as that. I'm aware that it's phoning home every time I start up the game, and I'm aware that they could revoke my access at any time, just because it was a promotional free copy that they don't want me to have any more. All things considered, I made a choice to play Portal for free, and will still not use Steam for anything that costs me money.
Mokalus of Borg
PS - I finished it yesterday.
PPS - I haven't yet played the bonus levels.