There's no clean cut answer here. It's all over the spectrum. Some founders get along great and some don't get along well. Obviously the better you and your cofounders get along, the higher your probability of success. I've seen plenty of bad cofounder dynamics kill companies, irrespective of the business fundamentals.
"It's like marriage, but without the sex" - I don't recall who said it. It's true - you get a lot of the stresses similar to a marriage, but there isn't too much fun unless things are going great.
It’s always positive outwardly, of course, but I’m asking about the side that people rarely see. It’s hard to know what’s normal and what’s not. And that’s a problem when you’re trying to evaluate whether someone is a good fit as a cofounder.
Consider Jamie and Adam from mythbusters. They can’t stand each other. But when they work together, things get done. Are most cofounders like that, or is that the exception rather than the rule?