If a system takes facing into account, you typically have the 3 sides as "front", the sides adjacent to the front sides as "sides" and "rear" as a single side behind. Thus, you have 180 deg of front, 60 of side on each side, and 60 deg of back.
With squares, you have 90 deg of each side. Rogues like that extra 30 deg of back to stab
When not dealing with facing, you have 3 possible flanking configurations. With squares, you have 4 possible configurations. If the target takes up more than one hex, don't even bother with flanking, you'll hurt yourself trying to make sense of it.
For vehicles on a 2 dimensional plane, hexes are great for depicting turning radii in regards to speed.
As previously mentioned, hexes suck for 3-dimensional movement if you want to maintain hexgonal aspects, otherwise you end up with Heroscape.