The ads for this movie focus so much on the pairing of Robert De Niro and Al Pacino that you'd think a) it's never been done before, and b) they were both still in their prime and not sad parodies of themselves by now.
If you think about a) for a minute (or go to IMDb), you'd realize that they've co-starred in two previous movies, both much better. They never actually shared the screen in "Godfather II", as they were the protagonists in the two stories separated by a generation (although technically, that means De Niro has played Pacino's father). And "Heat" was also much better than "Righteous Kill".
As for b), they've still got chops, and their acting is fine. It's just that De Niro is playing De Niro and Pacino is playing Pacino. The script seems to be written so explicitly for them that it gives them nothing that's remotely a challenge. They could both literally sleepwalk through this movie, and to their credit they don't.
While their acting holds the movie up, what drags it down is a lame script with a twist that seems so obvious that I almost thought myself out of it. You know the formula--set up one answer that is so obvious there has to be a twist. The audience will figure that out, so set up the obvious twist. The audience will think they're smart for knowing how it'll end, but then there's an extra twist at the end that surprises and delights them. Well, sometimes that obvious twist is so obvious that you know there's still another twist (which is usually completely nonsensical). But in this case, the obvious twist turns out to be right (oh, I guess that's a spoiler, but maybe I'm being general enough that it doesn't matter). And I'm left thinking, 'Okay, it was kinda nice to see two masters plying their craft, but I sat through all that for what, exactly?'