I've lifted both by myself and with a partner for years now. Now that I have experienced both I'd much rather workout alone. While a partner can be motivating and helpful when a spot is needed, sometimes partners can't be relied on. He can't go to the gym so maybe I'll skip this workout and do something else. I hated waiting for someone when they were running late too. I get my workouts done faster by myself and less chit chat. If I need a spot there is always a few guys I know there that can lend a hand.