I’ve used it before (though it was called photofly or something similar at the time). My results weren’t as good as I’ve seen others make, but I was stuck with a basic camera and limited photos at the time (with outdoor lighting to boot). I imagine with a good camera and a few minutes in a studio, you’d be able to get quite the result.
For the most part you’d just need to import your resulting model, and retopologize it. When I did it I aimed to shrinkwrap an existing mesh on top and so I did it externally in Softimage, but if you were going to retopologize from scratch you can get away with doing it all in zbrush too. You’ll probably want to slap it onto a body as well if you’re aiming to make a full character, unless you’re going to have clothes cover the neck seam. That would give you a nice edgeflow and good proportions from the start (along with the chance to fix any errors that it might generate - mine had quite a few holes in the back of the head along with pieces of the sky being captured in the model). So from there you can just subdivide and start to sculpt in any surface details that it might not have been captured. All the photos you’ve taken would make great reference for this part as well. Also add in things that might have been missed (teeth, etc).
You can choose if you’d want to transfer 123D’s generated texture over to your new retopologized model, if you’d rather clear it out and do your own version using Zapplink and all the photo’s you’ve taken, or if you’d want to polypaint the skin yourself.