I got this during the Christmas Steam sale, and only started to play it 2 weeks ago. Unfortunately my gaming time is rather limited lately (a couple of hours max / week), so it took me this long to finish the first
mission (kill Campbell). I started out trying not to kill anyone, but it took too much reloading and replaying. This is my fault, I often reload when a situation hasn't gone "perfect" in a game. For instance, if I alert a guard and he deals me a bit of damage, I'd often reload because I felt I could have handled it better.
But I can't really do that anymore, because with such slow progress I'll burn out on the game before I'm half-way. So yesterday evening I changed my gaming style: I will be a ruthless bastard, who will kill if needed to, and I will not reload scenes but live with the consequences.
Consequently, the game became a lot more fun and tense! And yes, I left a trail of wailing alarms and corpses behind me
I finished the original campaign last weekend. I rather quickly gave up on the above mentioned strategy (in short: kill if needed to, live with results) due to the fact I wanted a low chaos ending. And I didn't burn out on the game, it was a bit shorter than I expected (this is a plus for me these days). It is a brilliant game really. I love games like these in which the levels are just sandboxes for you to play around in. There's not a lot of scripting, but rather a very well made AI that reacts to your moves.
I started a new playthrough on the hardest difficulty, with the intention of killing just about everyone. But it's not really as much fun, nor is it any challenge. As Corvo, you're just too damn powerful really. If you want someone dead, it's really quite easy. Still, there's nothing wrong with being powerful. It's one of the reasons I absolutely love the recent Batman games. Still, the challenge could have been raised a bit I think.