But, his product credentials were impeccable, and I needed someone. So I signed him on. You can guess the rest of the story. My questionable subcontractor quickly turned into an unquestioned disaster. So, trust your gut.
Partnerships work best when the interests of both parties are aligned.
Your focus should be on helping the other person win as much as on winning yourself. They protect your customers as well as yourself. You don't want to fight over who services which accounts if there's ever a falling out between you and your partner. My own primary partnership dissolved after several years. The breakup wasn't pleasant, but, because we had a good contract, it was quick.