It's not that it's significantly harder to be good in the world of inFamous. It's just more annoying. As an example, you could spend an hour or two on the "good" side missions, which enhance your reputation and rid the streets of crime block by block, but if one of your electro grenades slips past an enemy and hits a pedestrian, a little red flash next to your "villain" will still let you know that you just became a bit more evil. Granted, you have to accidentally kill more than a handful of pedestrians to actually move down a rank on your karma meter, but these constant slaps on the wrist ultimately degrade the experience by needling all but the most cautious players. Pick the evil path, however, and you don't have to worry about perfectly aiming every one of your electro bolts: Unless you run around "accidentally healing people," you're not going to lose experience by going out of character.
If inFamous' morality system was the game's only misstep, it would still be a perfectly playable sandbox game. But like those constant moral decisions, there are plenty of other gameplay elements that only seem to get in the way of the fun. Chief among them is the A.I., which can spot you from blocks away and will take frequent potshots at you until you duck into an alley or take them out. The only way to avoid this nuisance altogether is to stay on top of buildings as you cross the city - an unfortunate option, given how rough inFamous' climbing system is. Figuring out what constitutes a handhold could be a mini-game in itself, and even if you have a clear route, you're still pretty much mashing the X button all the way to the top.
It's hard to fault Sucker Punch's ambition in inFamous - an open-world game where you have total freedom to become a hero or a villain would be pretty unique. Unfortunately, many of the gameplay decisions the developers made actually compromise the experience rather than bolster it. On the way toward a game with true moral ambiguity, I suspect inFamous will end up mostly as a cautionary tale: If you're going to let players choose their own path, you need to offer them more than simply "red or blue."
Bottom Line: While inFamous offers plenty of competent open-world gameplay, it's hamstrung by the same morality system that was supposed to set it apart.
Recommendation: Rent it. Even if you really dig the game's electricity-based combat, you'll probably tire of Empire City long before you've reached your full potential.
Jordan Deam nearly set his parents' cat on fire with a magnifying glass when he was 8. It's the closest he's come to a "karmic moment."