I haven't been in the position my self however, my father had a law practice for many years with a partner. His partner was egostistical and spent most of his time on image rather than work.
My brother owned a carpet cleaning business with a close friend. No falling out. My brother never gave the effort like his friend did and felt that it would hurt the business as well as the friendship. He bowed out.
Partnership is great if handled correctly, but get the wrong partner and all hell could break loose.
By the way, if your business is getting to a good point, enjoy it. Maybe you can find ways of partnering at arms length.