Fail. Headshot uses no energy and combos with Explosive Probe or Fragmentation Grenade. You should be using this whenever you use one of those, which, in the case of the probe, is whenever it's up.
Eviscerate is also very useful in combination with Debilitate and Shiv when you've got an add on you in that this sequence will often kill the add outright (unless it's strong), albeit less essential than Headshot. To some extent, use of these three melee abilities is a matter of situation and choice, but Headshot really isn't.