I'm currently writing a review of The Darkness II for the magazine. There's an embargo, so I can't talk much about it, but I'm fairly sure it won't cause too much trouble to tell you that I'm really enjoying it. In the normal run of things, that would mean I'd encourage you to check out the Xbox-Live exclusive demo that has popped up today. In this case, however, it isn't a good idea.
The Darkness II is a wholly narrative-driven shooter, and whilst the content of the demo doesn't contain huge spoilers, it doesn't do the final game justice either. A pop-up message at the start of the demo explains why: "This content is specifically designed for demo purposes, and will not appear in the final game in this order."
You're looking at a demo which mashes up two entirely different sections of the game in an attempt to make things feel more exciting. This wasn't a good idea. The full game's sense of pacing is excellent, and this jumbled-up version makes it seem overly scripted, and too action-heavy. It's exactly the same demo that I played at PAX - the same one that left me worried that The Darkness II might not live up to the original. As it turns out, it was the demo that was to blame, not the game.
If you're interested in playing through The Darkness II - and from what I've seen so far, you definitely should be - then I'd recommend leaving the demo alone. The chain of events in the full game is great, and playing it out of context first will dull the impact when you play through properly. If you're really not sure about whether you'd like it, then by all means jump in and have a quick go, but for those of you already excited about playing it, resist the temptation to get involved.
Trust me on this one. I think the wait will be worth it.