I've used the Microscribe 3D digitizer so here's my 1 1/2 bits.
It all breaks down to how well you draw the contours on the model that ur digitizing. The process is bloody tedious especially if you're going n for a high detail model.
The key is to trace contours on the model with perfect precision and trace the points accurately( Aaargh....just think 1123...1124...1125...112?..shit did i do that point.....heh heh.....it's not thaaat bad but u get the idea.
Another major drawback is the damned small arm of the digitizer which doesn't work too well with large models.
U would have to digitize half the head first then turn the model around and start doing the other half then the frickin arm of the digitizer won't frickin reach the frickin ear and u gotta do that separately and remember all this is to be done without any small movements of the model otherwise you would have to go to mesh cleaning land for a long long vacation( GEEZ !!!)

But all said and done with a bit of practice and loads and loads of the frickin 8 letter word P-A-T-I-E-N-C-E u could get some reall cool output but I rather go with good old talent, by hand modelling the object (it helps improve ur skill whereas the digitizer can become irritatingly monotonus and mechanical after some time and anybody can do it.)