I buy my games. Almost all of them. Almost. Hell, I even buy games that I once pirated, doubly so if I though the game was good and that, therefore, I owe the developer for the game.
But some games I've not felt inclined to pay for (and some of those I actually did pay for and wished I hadn't) and it was only around three hours in that I saw through the charade that was the demo to the rather dull core of the game itself. The original Call of Duty was one I bought and wished I hadn't; the demo was all the best bits of the game, or at least until I gave up due to boredom. I found Guild Wars equally dull. Even electronic purchases have served me up with games I just found boring: Galactic Civilizations, for example. I find that I'm wary of buying games just in case they're crap and look for ways to get a real sample of the game before buying it. Demos, largely, just don't cut it as it seems that they're deliberately slanted to contain the best of the game and sometimes the only good content...!
But if it's good, I always buy it eventually, or something else from the same studio/development shop if the original is no longer available.