I use GLB exclusively, and I have loomed it up to Powerful so far. It is a very situational weapon, moreso than the flamberge:
1v1 against pretty much anyone that can block, I lose.
1v1 against a shielder with any semblance of facehugging skills, I lose.
Standing on top of a ladder/ramp in siege defense, I've gotten 15+ kills in a round
Free-for-all situations (i.e. around a flag), I can go around 1-shotting enemies that are engaged with my allies
But the fact that it it's slow as dirt and can't feint means that I lose almost every 1v1 fight. The flamberge can actually be an OK dueling weapon, since you can feint.
Also, it's worth noting that the regular Long Bardiche is way more OP than the Great Long Bardiche. It isn't unbalanced, does only 1 less damage, and is 2 speed faster, while still having very good reach.