Currently my training partner is my husband (a former personal trainer), however before that, I did chummy up with a young woman at the gym and occasionally we would train together.
While I believe that differences in ability does not matter (ie., one person stronger than the other should not hinder each other's workouts) -- the same "work ethic" must apply between partners. The woman I was working out with, was not timely nor committed. I would spend time waiting for her -- either she was extremely late or conducting a social coffee clutch. It was more of place for her to go rather than really putting in a good workout.
I like having a workout partner for some of my workouts but do like to train alone as well. Yes it motivates you and provides a good spot for forced reps. For me, working with my husband, it gives us time to be alone together...no phone, no bosses, no child, no dogs! Just us.