Despite being one of the greatest multi-skilled entertainers ever, Davis is vastly underrated as a singer.  A major part of it, in my eyes, is that his attention span was short, he did so many things so well that he could not concentrate solely on his singing and his music for any length of time.  Another is that his studio records were awful and his live recordings often contain so much shtick (see his Coconut Grove CD) that they become sickening.