I really enjoy Quake. Don't worry, I haven't liked Marathon any less, and Quake hasn't surpassed my passion for Marathon. But Quake just flat out kicks ass. Pretty challenging puzzles, a balanced arsenal (well, except for the lightning gun or whatever), killer enemies, hardcore gameplay, grabbing environment, pretty damn good graphics, and so much more.
I tried Q2 and Q3 to see if they were any better. They weren't. These sequels are pieces of crap, on par with Unreal 2: The Awakening and Sin, two MORE pieces of beta test public release crap. The disappointment and lack of good gameplay in these two sequels really made my mind awake (or should I say QUAKE) to the fact of how much these sequels suck, and how many other modern games suck.
Anyway, I got the full version. I can't remember specifically what site I went to to download the demo, but I remember going under a google search to find it.
It's a very, VERY good game. I wonder how the hell they screwed Quake II and Quake III: Arena up.