I was playing it last night and I must say I am completely enthralled. It's not a 'mile-a-minute' kind of gameplay. It is often quite sedentary and considered. You will have to think through your investigative actions and each decision is actually quite nerve wracking as it can stymie your investigation, or make you come off as a sloppy detective at the end.
So far I've come across a couple of investigations with alternate outcomes, depending on how well you investigate, and whether you resort to violence. I'd say there seems to be a fair degree of replay value in trying to complete investigations as best as possible, looking for missed clues, re-attempting previously failed lines of questioning, and generally being the best detective you can be.
I haven't had a chance to test out the social integration, but it appears promising - you can use some of your investigative intuition points to see what other players have done during the same case, and perhaps plan actions based on that.
The facial animation system is astounding; the most realistic I've ever seen in a game. It is also core to the gameplay as you will have to observe characters to discern if they are lying or deceitful - it works incredibly well.
There are also action packed moments as you will often have to engage in chases or shootouts with bad guys, and these make for a great change of pace from the core investigation gameplay. There are also plenty of side missions to keep you occupied aside from the main story.
I can't comment on the amount of gameplay hours you'll get out of it, but from my first impressions I think it will offer many, many hours of detective awesomeness. The cases seem to be getting pretty meaty and they take a good while to complete, especially if you strive to pay attention to detail.
All-in-all, from my first evening of gameplay, I wouldn't hesitate to recommend it. It's unlike any other game I've played and if you are willing to embrace the detective elements of the game, and it's fantastic storytelling ability, then you will be well pleased.
Anyone looking for another GTA - this is not that game.