When you get a couple of them stacked the EMP can make a difference, but on a single gun its not much. A single ion cannon shot every 2 seconds doesn't do much.
The big advantage compared with an ion cannon is that the ion cannons get completely negated by shields. In this sense the HVD is useful under most circumstances, so I often use it for that purpose, if I can't get my hands on some modded kinetic+emp gun, DME and GKS has a few, IIRC, which are slanted more towards damage with some minor EMP.
Not very good for killing things in a hurry, though. And having a Wolf /w Ion Pulser (or a Tempest) or a carrier with Claws or Thunders in your fleet is more efficient for countering shields and inflicting EMP. If you have either, having a HVD on your ship is more insurance than anything else.