Restorative Mist is, apparently, the MEC version of a medkit. How does it compare to medkits, though? How many uses do you get? What's the radius? How many hitpoints are healed per use?
It is the same health restoration as using a medkit (4 or 6 depending on Foundry project), but it applies to every soldier in it's 3 tile range (including the MEC that uses the skill). You cannot use it to revive/stabilize soldiers who are bleeding out.
When you first get it you may use it once per mission, if you pick up Expanded Storage as your Captain promotion you get an additional use.
Here is a screenshot of Restorative Mist in action. It's a little hard to see the grid, but the lone soldier at the top of the stack on the left is 3 squares away in a straight line from the MEC using Mist.