Until a couple of months ago, I only had an onboard graphics card. I couldn't really play anything newer than Deus Ex.
I only got Portal when Steam was giving it for free some time ago (I know, I know). I installed it and and started playing, but only to see if my PC could take it. It barely could. I thought, "Well, this is too much lag... I don't think I'll be able to play it properly... I should stop and play it later when I get a decent graphics card..."
About 4 hours later, I was listening to Still Alive and wondering what the hell had just happened.
Seriously, even if the game was terribly laggy I just couldn't stop. This is Portal for you, people