Yeah, we both got out. It was about 2 minutes later that I said he can drop me off.
To be clear: I'd made up my mind after the run-up I just was finding the right spot in the conversation to voice it. I don't even know what can cause an engine to run that rough. At least nothing I've ever experienced in any airplane I've flown.
I'll admit, I probably would have bailed earlier except for that fact he was a nice guy and I felt bad. I know that's not a great reason. But he had taken off work just to go fly. And here I was essentially saying "thanks, but your plane is a busted piece of **** that I don't feel safe in".
In the end common sense prevailed and I left fairly gracefully, but it took longer than it should have lol.