It's not an easy question to answer without asking a few questions.
What do you want with it? If you're after it from a background perspective (like I was) it's useful. If you're playing games of Dark Heresy, it's interesting in that it dramatically increases the power level of the game. Ascension is perhaps the closest to Inquisitor in that you can now actually play an Inquisitor and his closest associates, rather than a cell of Inquisitorial agents. It means that the power level (and influence) of your opponents has also been given an increase. There's some really very interesting stuff in the Ascension book, but it depends on your tastes. I think it's definitely worth trying to get hold of it, though!