By the time the credits rolled, I had lost all respect for Quantum Conundrum. It seemed like it had potential to be a decent little game, even if it was riding on Portal's coattails. The sad truth is that it does very little to be an enjoyable experience outside of introducing some nifty mechanics. The plot, music, and visuals are all basic and boring, while the puzzles barely offer a challenge (and most of the time, just offer hair-pulling rage when they demand tight precision that the game is not built for).
I have no right to tell you what you should or shouldn't buy, but I do urge you to be somewhat cautious about this purchase. The disappointment won't hit you until the game is over, so I'm not sure how helpful the demo will be in deciding whether or not this is worth your money. In my opinion, the only thing Quantum Conundrum does well is create the illusion that it's going to go somewhere, but it never does. The only reason I felt compelled to keep playing was for the point when the tutorial ended and I would be set loose to have fun with the dimensions, but the game ended before that happened. If you must buy it, I'd at least recommend waiting until it goes for crazy cheap on Steam. Like, under $5 cheap. I really don't feel it's worth any more than that.