I played around with Kilo pumps, the water version of the fan. I was actually quite impressed.
They are kind of like weapons with "added effect: wind damage" except they trigger more often and for FAR more damage.
With my kilo pumps they added +30-40 dmg per punch (for a monk who hits for 30-50 in the low-mid level range that is significant). I have heard the damage is constant on them, so on a heavier hitting 2-handed weapon you still get around +30 dmg added, so it doesnt seems as drastic.
I never tried the mega series of pump/fan/battery, but heard the dmg was like +50-70. The scary thing is that if these triggered just a little bit more often they would one of the best DoT weapons in the game.
But for now their only real use is on melee resistant mobs like slimes and elementals, or extremely high defense mobs. The damage they add is constant so the added elemental dmg will easily surpass your weapon base dmg.