Original question: yes, you can fire Overwatch and then run D6" in any direction.
Guys above are right that a simple 3.5" longer charge distance isn't that amazing in itself. However, there are a number of other tricks you can use here. For example, it specifically says that our special move is a Consolidation - and BRB p53 states that units making a consolidation move are not slowed by difficult terrain. You know who is slowed by difficult terrain though? Charging units, that's who! They get a -2" to their total roll. A unit charging through difficult terrain is also forced to fight at initiative 1 - for the first time in history, Tau will strike first in melee!
Another thing you can do is to walk behind obstacles. It is not possible to charge through friendly units or enemy units that aren't being charged. If the unit charging you is on large bases, you can retreat your tiny models through narrow passages that larger models cannot fit through and suddenly he cannot reach you at all.
Furthermore, maybe your Overwatch was extremely successful and wiped most of his unit, so that your opponent no longer wants to complete the charge (because his unit is now likely to get wiped out). Then you can consolidate towards him, making it more likely he will succeed and die.