Don't Quit. 6 months is just too short of a time. No matter what you do afterwards it will always leave a question mark why you left so early, in particular when the start up has so much traction. That the founder sucks is too bad. However, to get along with people on a professional basis is way different than getting along with them on a private basis. Think of the army. Imagine people would quit just because they dont get along with the General. Persevere and try to stay at least a year...