Well, I finally finished inFamous and, a few niggles aside, I loved it.
The game is as I described above, a force lightning/parkour version of GTA!
You can influence your characters karma at certain break points, so finding pedestrians have strung someone up, you can leave him to his fate or set him free. Your karma choices determine which powers you develop and also (in a limited way) affect the story as it progresses. Your appearance also changes, make evil choices and your skin fades to gray and you develop nasty scars on your face. Also, pedestrians will throw rocks at you!

Make good karma choices and pedestrians will throw rocks at your enemies as you fight them and ask to take your picture!
The game is split of three islands, with a reasonable speed of progression across them. You have a range of powers (15 or so) that are learned by your character as the story progresses. They range from electric grenades to the power to call down lightning storms. The basic powers can be upgraded, once you have earned enough XP, to increase the scope and force of the power. Grenades split into 3, 5 or 7 on landing!
The story, while not massively original (gasp!), plays out quite well, expanding the enemy ranges just as the last is becoming predictable; introducing new characters and motivations at well spaced intervals. It also ends quite well, although I won't say too much.
My only niggles are
- that the balancing of the XP required and the speed at which you earn it could have been better. On my first play though (on hard level) I completed the missions and needed 8000 xp to buy the last weapon upgrade. As a killing a standard bad guy earns 10XP, you can imagine that is was a bind to do it. I did, but I'm a trophy whore!

- The building models are re-used too frequently. Sometimes it's hard to remember where you saw a mission on the map, because the same round building is reused on all three islands. There is an in-game map with way settable waypoints for that though! But a bit more variety would have been nice.
Overall : 9/10