I think it's important to be able to separate business from friendship. The worst is when you're romantically involved with one of your start-up partners. Never do that.<p>It also helps to start companies with people who are exceptionally laid-back, but you have to also realize that with that sometimes comes laziness. It's hard to find teammates who are both laid-back and highly driven/motivated, but when you do, they're solid gold. Whether you're friends before/during/after your endeavors is immaterial at that point.
I started Joystickers with 2 other founders and we had to part ways with one. It was messy, but we're all still friends.<p>I bought an investment property with a different friend and our friendship has definitely suffered. We don't hang out for fun nearly as often as we used to.<p>It really depends on the dynamic between the friends. If you guys are comfortable butting heads without taking it personally, then go for it. Think twice otherwise.