I find it most useful during certain Levequests where you have to fight large numbers of monsters, like the one at Bloodshore with the beatles and roselings. Anytime you get surrounded, it works really well. The range on it is actually pretty huge. But to use it against something you have targeted, you need to back up about 5 feet.

Also of note, Arrow Helix is the exact same way. Also, another great time is when you're in a fairly isolated area where it won't accidentally hit things, and your target runs for another party member, you can use it then as well. It's all about knowing the properties of an attack, and thinking about uses for it, THEN working it in practice. I thought Iron Tempest sucked until I found ways to make it work. After that, it serves me well. A lot of times I'll duo during a leve since things come in pairs. If my partner isn't standing some place near for another AoE, but needs some help, I'll also use Iron Tempest then as well.

But, I will say... It could use the hole filled in as it does subtract from its overall usefulness.