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I find "reality checks" to be a great benefit of having a co-founder. The best of us spend time pursuing ideas and tangents for our startups that really don't matter. A co-founder can help identify and keep those incidents to a minimum. Of course this works both ways.

Reality checks are best provided by customers, or potential customers.

It depends on your working habits. Simply having someone else to report/share progress, ideas, etc to can make a huge difference, especially if, as recently discussed on HN, you're inclined towards the manic depressive. Having someone else there really helps during the low periods.

Partnerships have a good track record. I think they add some consistency to the performance.