Eight or nine. The only disappointment for me is that the game is not as long as I'd been led to believe by some previews and interviews. The single player campaign was about one and a half times as long (for me, anyway) as with the first game, while I'd been expecting twice or more. But it was amazing in every other way.
It also seems to be easier in some ways (not a criticism, just an observation). The deadly energy balls are replaced with beams that take longer to kill you, and what timing and quick action puzzles there are are more forgiving. I suspect this was done mainly for the benefit of console players, but I actually welcome these changes on PC too. Puts more focus on thinking.
although we do still have a time machine, so I wouldn't completely rule out the possibility.