I made my first mummy down bag with a zipper in the middle, found it the best solution: easy to get to with both hands, no zipper in your way when you sleep on your side, easy to keep open or closed to regulate the temperature. The con is that the top part and puller, if not properly protected, hidden under some kind of flap, can get into chin, mouth or nose quite uncomfortably; needs a good anti-draft flap too.
Some bags have zipper starting on the side and ending on the front (diagonal?), probably a good compromise.