I've had one for a few years now, Triblader is it's owner... What's to say? I acquired mine via a rather *ahem* awesome deal from Kajex.
It's another tool in the toolbox to me, albeit it a VERY powerful tool under very risky circumstances.
But again, as most know about it, Dark Flow's full power it only tempered by your immediate vulnerability, to unleash it's true power you must be near death.
To achieve near death quickly at will without the help of traps or mobs, you'll want a good beserk special weapon to lower your HP that does not rely on a target, most people use a Girasole/Inferno Girasole or even Lavis Blades to achieve this at will.
For some, this is a exciting and challenging prospect, for others, a scary "knife edge" of trepidation, as you become a literal "glass cannon".
Dark Flow is a weapon that does best when you've got others in a group to screen your position and keep mobs off you while you annihilate everything within Dark Flow's "energy wave" range.
Outside of that, you had best be light on your feet with one finger on the hotkey for a trimate.
Bottom line, it's the strongest and one of the best "ranged weapons" for a HU period, get one if at all possible... In the hands of a HUcast it is devastating.
Even if it's just to see it's power for yourself for a while, try to borrow one if possible, as it's playstyle is rather hectic and quite challenging when "Flowing". You may or may not like it...
Ahhh, another point I remembered, you will also have to be rather weary of FO's and other players using resta and stay out of their resta range, as it can stop your "Flowing" rampage in an instant.
Hope this helps Kira, good luck in your quest to acquire the "sword of ultimate power" here at Ultima.