Chances are that you've played a demo of a game before.  Also, chances are that you've played the full version once it was released.  There usually is no difference at all between the two, whether it be good or bad.  The game that I have recently had experience with was Fight Night Round 3.  I was so excited to play the demo, so when I got it, I was disappointed by the sluggish controls.  I thought the game was very slow and it really wasn't as intense as its predecessors.  I checked the forums to see if anyone felt the same way and found many who did.  As I read over them, the number one reply was, "Its only the demo."  Well, this is true, but how often have you actually known them to change the game from the demo to the final product.  Never.  You read everywhere about people complaining about how demos have something wrong with them and others replying that its only the demo and they'll fix it when the full game comes out, but lets face it, they never do.  So if it was up to me, I'd take away the saying, "Its only a demo," because while it may be only a demo, its almost an exact replica of what the finished product will be like.