Me and the Missus played a leisurely hour or so tonight. First impressions: I love the atmosphere; the environments are truly beautiful; and I agree with Paddy (I always agree with Paddy) that the mystery solving and flashback mechanics are original, innovative and intuitive. I'm loving it and I want to play more.
Only problem, my PC can't handle the amount of loading in the open world. If I hang around in one place it runs beautifully, but if I take just... a few... steps.... over..... there....... it stutters and hangs for a literally minutes until all the new assets are loaded in. Tweaking settings and freeing up harddrive space does not avail. Sometimes I think there's nothing wrong with discreet areas and the occasional loading screen.
I won't let performane issues put me off this very intriguing game though. Going to give it a shot on Mrs Behemoth's PC, which is 64bit and has more RAM, but just onboard graphics. Might not look as pretty, but will hopefully be smoother. If not, I'll endure it on mine, that's how much I'm liking it so far!