I have been playing the Conviction demo and i must say that i am impressed, my only issue is that it's a little short. Im loving the 'mark and execute' and 'takedown' systems, they are very welcome additions to a great game series. Fisher has become even more badass in this new outing, lets just hope that the full game is as good as the demo.
Usually demos are just that demos. Not the final product. They rely on the feedback from people playing to do any last minute changes, bug fixings, and all that.
I stay away from demos. I usually wait for GameTrailers and my GameInformer to show me the game and review.