there is a error in the smoke... on the right you can see the smoke stop in a line. this breaks the natural look of it. you should had noticed that before merge with the model. apart from that is a great shot. love it.
It's real smoke - just shot separately. Get a couple of incense sticks and light them next to each other, about an inch apart (so the two smoke plumes intertwine with each other). Then it's just a case of comping it in with photoshop.