Playing demos is important. I play a lot of them. A mostly complete list with all demos I played in all platforms.
There's a note for the cases in which I played the full game after ('PLAYED FULL GAME') and for those I bought and/or played JUST because of having played the demo ('CONVINCED BY DEMO'). Some of them are full games I tried at friends' places, which is functionally a demo ('TRIED BUT NOT DEMO').

This includes demos, betas, alphas, trials, and everything. I'm only using the word 'demo' for all for convenience.