I managed to sneak in a few minutes to play the demo.
Only one thing to add so far: Throw away all the inFamous gameplay videos on the net.
They are boring because they gave the impression that I have to haul many lightning balls just to kill a few grunts. In the demo, your attack is powerful (especially the electric grenade).
The other impression I got from earlier trailers was that the grunts were stupid and boring. In the demo, you can in fact get your *ss handed back to you by just one "foot soldier". Some of them explode on contact (one-hit kill), a few are rather sneaky and managed to kill me by using the environment as cover. Didn't see them until they were right below me (Got headshotted at gun point by something that look like a sniper rifle).
Bottomline is you need to watch the environment for threats. You can't just run around and focus on the game objectives only. You have to be aware of your surrounding. The city is dangerous, even for a super hero.
The last thing I noticed right away was the controls. Cole reacts to the environment contextually and is very responsive. It's easy to climb, latch on, and jump onto any structure depending on where you stand.
Good job so far. I haven't seen the boss in the demo yet.
EDIT: It does not appear to have GTA4's visual problems (Aliasing is hardly noticeable for me here. Screen is not blurry. Not sure about framerate since I have not gotten into huge battle yet). The exploring is also more flexible than Assassin's Creed.