So after chatting with Cider yesterday about some of my experiences with my Specialist/Biotechnician, he advised that I post some of the info up.
With all the high damage weapons you get, the listed damage on a leech gun can be a bit underwhelming, but its effectiveness in certain combat situations becomes apparent quite quickly.
I'll start with how a leech gun does damage. The animation gives the visual effect of a channel, which is basically what it is since the leech gun does its listed damage to a target, and any damage that is not resisted is then returned to the attacking player in Armor or Health. No worries though, you can feel free to move about as much as you like (I know just out of WoW-ish instinct I was wanting to initially stand in one spot while using the weapon), just like with any weapon in TR. Being at full health does not prevent your leech gun from doing damage, however it can be misleading since (currently) it does not display the damage being done in the Scrolling Combat UI, you just have to end up watching the unit's lifebar and see that steadily go down (which should be fairly apparent, since Leech Guns do damage every second).
So the important aspect of a leech gun is that it leeches armor or health from your target and transfers that in exact amounts to your (at least at the rank one of the ability, I don't have higher right now). Probably the primary reason I'm writing this up is because the mechanics behind leech guns feel/appear a bit buggy when it comes to draining a target that has armor.
So, we'll say your Specialist is off fighting a group of units and you're armor is getting dangerously low. If you equip your leech gun and start using it on a target that has no armor, or its armor has already been depleted, then you'll only regen health when using the leech gun. If your target still has armor remaining or has full armor when using your leech gun, YOU WILL STILL CONTINUE TO REGEN ARMOR WHEN ITS ARMOR IS DEPLETED. This is what I personally am led to believe is the bug with the leech gun right now. Since normal leech gun behavior is to regen what stat is being drained, it seems that the game doesn't realize that the unit is out of armor when its being drained, so still regens your character's armor. With how important armor is right now, this can pretty much make you near invulnerable. :)
A leech gun can crit and cause overkill, although for the longest time I hadn't seen this happen so I'm assuming the weapon may have an innately lower crit rate than other weapons.
This is all I can think of at the moment. If you have questions, I'll be more than happy to answer them or test them out.