Everytime I see someone holding a charge, I shoot at him with sentenza :)

I've seen people successfully take advantage of the Flourish's charge attack in Lockdown, but as Demonicsothe said, they usually come from the bottom of the screen where most wouldn't see it coming. Knowing that the Final Flourish's normal attacks are powerful, the charge shot is a hell-of-a-lot more powerful, possibly taking out a Recon with no heart Pendants, holding a maximum health of +15.

With the Recon's universal CTR reduction, you'd be surprised how many strikers you can kill with a surprise charge attack. It's rather amusing. The Flourish charge has a deceptively long range with a powerful end stroke, and generally kills anyone unlucky enough to get caught with it. However, it's really easy to dodge if someone sees it coming, and the recovery after the charge attack is poor, which can easily get you killed. It's a very "All-or-nothing" charge attack, so think twice before you use it, or just use it as a desperation move/last stand gamble. The best way to use it is when the enemy doesn't see it coming, or are otherwise distracted by your teammates.

The weapon's charge does excellent single target damage single targets in PvE (Alpha Wolvers begone!), or even large crowds if you learn how to use the last stroke of the charge attack correctly. Much of the weapon's charge attack is about learning its range.

Long story short, for both modes, if the charge connects, it kills. If it doesn't connect, hope that nothing decides to whack you while you're left open. In PvP, you're more likely to be attacked if you miss, since other players can be a bit more brainy than the AI of the Clockworks monsters. Of course, if you miss against a Devilite... Still, I'd recommend you use it for PvE. The charge attack may get more use there, as you're less likely to get punished for missing with the charge.