I prefer an oval handle - seems to allow my thumb and forefinger (well, fingers) grip to 'sit' better. More important for me is the thickness of the handle, irrespective of shape, for comfort and getting the right feel.
So many very thin handles around at the moment - losing weight in the bat - part of a trend towards enabling bigger blades for seemingly low(ish) weight, perhaps? How effective are they, though?
I think the idea that a thicker and therefore stiffer handle is more effective at transferring energy to the ball (ie allowing it to be hit further/harder), makes a lot of sense. Paul Aldred espouses this, eloquently, in some of his Youtube videos.