I think I played the demo disc with Klonoa 2 maybe 100 times before I finally picked up that game a few years later. It was love at first sight and it's still one of my favorite platformers EVAR. If Klonoa 3 was announced I would probably die.
I also played the demo for Dark Cloud A LOT back in the day, and didn't pick up the game until 3 years ago. I had already played through Dark Cloud 2 by that time so the first game didn't really do that much for me in the end. But I'll always remember dat demo!
I also had a couple of PS1 demos with stuff like Little Big Adventure, and another game that I cannot for the love of me remember the name of. I want to say Johnny Bazookatune, but it may not have been that. I'll google straight away.