I've never cum hands free while sucking cock, but sucking does get me so aroused that I can easily cum from stroking myself. If I'm with a guy and sex is just not doing it for me, I always know that I can finish if I blow him. My last FWB knew this and he used to come over to my place to feed me his dick, knowing how excited it made me, and I think it made him more excited seeing me get off on sucking his dick. He always reached orgasm as well, so we had a great symbiotic thing going. Sadly, he moved to another state. It was great while it lasted.