I played a batch of demos recently and here's what I thought of them.
Limbo
This was a great demo. It really made me want to play the game. I already knew what to expect out of Limbo, so I wasn't surprised by anything that was in it. But it did give me a good appreciation for the tone of the game. I plan to buy this.
Dead Block
This was kinda like a really strange and poorly designed tower defense game with zombies. The problem with it is that the design seems kind of bad, the goofiness isn't endearing, the controls are shit and the camera is shit.
Catherine
I didn't really have any idea what to expect out of this game, much less its demo. It's a block puzzle type game with relationship segments and LOTS of anime. It was actually really really fun and made me really want to buy the game. If you have PS3 or 360, I would definitely suggest downloading this demo. Once I get the money I'm going to buy it.
Beyond Good and Evil HD
This demo was also really good. You can definitely feel how it's an earlier last generation game, but the story that I caught was really good and the characters and voice acting was enjoyable. I plan on getting this game as well.
To recap that's 3 games I'm going to buy and one that I'm sorry that I ever touched.