good is relative...a cube won't harm you much anymore if you drain 70% of his weapon power and shield power...but his HY torp is still dangerous. the surplus power allways helps you in return. another problem i see is, that appart from stfs there are hardly any enemies that live long enough to see the end of a siphon.
definately better suited for PVP, but far from useless in PVE, just that there are not many opportunities to use them appart from stfs.
Try running a full polaron build with the Jem'Hadar set. Next to the 7.6% extra dmg the 3rd bonus also is a shield killer. Try running GW3 and Vent Theta Radiation with that and watch those NPC suffer :p
In order: Gravity Well 3 -> Anti Proton Sweep (3rd bonus) -> Evasive Maneuvers -> Vent Theta Radiation -> Torp spread 2.
I do this in an Intrepid and it works pretty good.
Would put more sci feel into it, but atm it's my sci officers for my odyssey, so I just threw it together with a MURSV as a troll-boat. Turns out I got trolled by my own ship when it turned out to be incredibly effective, and as a result, was upgraded to a FRSV. Whadya know XD.