I generally will play the full demo if I am thinking of purchasing the game. Quite a few I uninstall within 10 minutes.
Yes, I have also played the full demo to see how far I can get, but when I do that I skip all dialogs. If within the demo I can no longer progress (Granny type games) I know not to purchase as I'm already stuck.
I have a lot of demos on my notebook that goes to some doctors with me and it helps to pass the time in the waiting rooms. I purchase and play what I like on my desktop.
Linda