There was a time when games really did offer you the opportunity to try before you buy, Doom gave you a full third, and by no means the worst third , of the game for free. When ID made the next big change to their games and released Quake they used the same model, giving away the first episode (of four this time) away for free and many other games developers of the time followed the same model. Today this practice has died out (small independent developers still keep it up, but mostly for casual games) as it wasn’t long before rather than releasing a large chunk of the game they released a single level and the demo was born.
These single (often training and the first level) demos were not too bad though. They let you know if the game made unreasonable demands on your system and usually gave you enough of an idea about the game to decide if you would enjoy it enough to buy it. I could list many games I bought on the strength of a demo but a few really stick out. The Liberty Island mission that was included in the demo for Deus Ex was enough to prompt a purchase, as was the single track available in the demo of Re-Volt. Some developers went even further and created an extra level which was not part of the game to release as a demo to entice potential buyers, the demo level for the original Half-Life was an example of how to do it right but as I will mention later Valve have somewhat fallen from grace in this respect lately…
So what of now? In my opinion far too many games are not releasing demos at all, there has been no PC demo for Mirrors Edge despite a demo on the consoles and the fact that Epic’s Unreal 3 Engine should mean it is about an hours work to convert it, S.T.A.L.K.E.R.: Shadow of Chernobyl never had a demo which considering that no one could tell if it would even run on their computer at the time was a crime and Bethesda didn’t bother making a demo for Fallout 3 (which considering how buggy it is would have been appreciated).
Even Valve, despite their efforts with their first game, didn’t bother to release a demo for Half-Life 2 [drugcrased has pointed out to me that this isn’t true there is a Half-Life 2 demo here]. With the DRM of today’s games preventing refunds the lack of a demo should be unforgivable, how do you know that you will like the game they expect you to part with £29.99 for? How do you even know it will run on your computer? Do you think that all the people who had ridiculously poor frame rates in GTA IV would have bought it had there been a demo available? If every game had to have a demo by law before they could sell it then I think developers would be more careful about optimising their games before release.
Nothing I have said so far is particularly new, but lets go back to Valve, the developer who at one time made an extra mission just for the demo, what are they doing with their latest game Left 4 Dead 2? Well they have taken the idea of using a demo to sell the game one step further, they are instead giving early access to the demo to those who have pre-purchased  the game, instead of the demo selling the game, the promise of the demo is selling the game. Valve, You’re doing it wrong. The people who pre-order it don’t need a demo, they have already decided that the game is worth buying or you wouldn’t have their money. You should be giving the early demo to people like me who are still unconvinced that this game is going to be worth buying. Besides, if this is anything like the demo for the original Left 4 Dead it isn’t really a demo at all, in my opinion a demo should allow you to play that one level they give you over and over again until you either buy the game for the rest of them or just give up. Left 4 Dead gave you one campaign but it expired when the game was released, forcing you to buy the game or stop playing, since its release just under a year ago there has been no demo for Left 4 Dead, what if you were busy in the three days the “demo” was available? Well they did allow you to play the full game on occasional weekends but the servers got so busy it was a waste of time, most people (including me) purchased the game without playing it as we have to do with more and more games these days, I regretted it and if I am honest am regretting more and more game purchases these days purely due to a lack of demos.
A thought to end on; Would you buy a car without a test drive?
 Though it misses my favourite level which was Containment Area from Episode 2.
 Note the Purchased, that means you pay now rather than on release and can’t cancel, which I am sure breaks European law but as no one will stand up to them they can get away with it.