I'd want to make sure that whatever the outcome I wanted from the partnership, all of the partners wanted it just as much as me. It would be a mistake to enter into a partnership where something was of vital interest to you, but only peripheral interest to the partners.

Beyond that, it's just the usual factors about reputation, quality of work, trust, price, and so on -- factors that would enter into the decision of any important relationship. But being "just as vital to them as it is to us" is at the top of my list.

So, what happens if it isn't just as vital to them as it is to us? Then perhaps it shouldn't be a partnership. Perhaps it should be brought in house. Or perhaps a different partner should be sought. Or perhaps it should be structured differently -- perhaps a joint venture -- so that it can become more important to the partner.