My skyranger got iced by aliens on the second mission I did.
I also put my base as central as possible (Iraq), only to discover that the fucking aliens were busy trampling on brazil
I've played it for about 3 hours and it's already paid for itself in terms of "RRRRRRRGH" tense gameplay (And being unable to figure out how to overwatch)
Overwatch blows goats. It's worked out on how many TUs you have left as a percentage, multiplied by your reaction percentage, compared to how many percent (total) an alien has left of their TUs. You only use snap shot (aliens use auto and super-squad-sight). Maybe as an actual TU count comparison, or as a percentage, I forget which. It's probably actual modified TU count, so fast aliens are harder to shoot. More TUs on a soldier's stats probably helps as much as a few extra reaction percent (from memory). So at least it "kind of" auto-trains as your soldiers get more TUs from doing their normal stuff anyway. The reaction firing soldier then drops down the Reaction Thingy list as many TUs as the shot used in any further comparisons against that alien's TU total.
Here's the actual info:
http://www.ufopaedia.org/index.php?title=Reactionshttp://www.ufopaedia.org/index.php?title=Reaction_fire_formulahttp://www.ufopaedia.org/index.php?title=Reaction_fire_triggersIt can work if you have a couple of guys facing the right way, that just crouch, outside of smoke, and don't move or fire ever. It's better on terror mission where stuff just wanders into view while chasing civvies. Just saving enough TUs for a snap shot isn't enough (you'll start at about 30% TUs depending on weapon, divided by your reaction percentage, so you'll be at like 10 TU on the "Reaction Thingy" list, which isn't even enough to fire, and the aliens WILL shoot you first). The soldier firing must be able to "actually see" their target, so no super-squad-sight is used (AFAIK) for Xcom soldiers.
Reaction stats only ever get boosted if you actually make some reaction shots. Pistols/laser pistols and laser rifles will give you reaction shots sometimes if you don't move an inch and something wanders on past. Plasma pistols/rifles and Heavy Plasmas are ok too, just for accuracy and reasonable snap fire times.
There's very few times where relying on reaction fire is good to do when you can just pop smoke. Turning two soldiers into semi immobile "maybes", without cover, usually gets them killed.
If you give them plenty of armour, it can work. Just sit everyone outside a small ufo's door with a low-tu snap firing weapon armed on everyone you want to have good reactions with. End turn. Keep ending turn, keeping full TUs on everyone. Either an alien will come out and fry you because of your crappy reaction and aim, or you might get some reaction shots in. Maybe. Congratulations. That soldier just got "sit there and do nothing" duty for the rest of his life. It will be short. Actually easier on Genius/Superhuman, because there's usually more aliens in small ufos, so they pop out more.
You can also do it from the edge of smoke (one or two squares in), just to make sure the alien has moved a bit before you see him (you might be higher on the Reaction Thingy than him by the time you both spot each other, because he's moved to see you, you've still got full TUs, even with crappy reactions dividing it). This is the "normal" way of getting reaction shots.
It can also work with flying armour. Fly up, pop smoke on top of the skyranger. Fly near the edge of the smoke, crouch mid-air and wait. Keep waiting every turn. Probably best to use a pistol or laser pistol for this (assuming you're training, not killing). At least you'll have great vision lanes and a bit of smoke protection on that z-level. Works with more accurate weapons if you actually do want to kill something with reaction fire. Might be best to pop smoke on the z-level under you as well (on the skyranger's wings and on the initial items square) for a bit more vision protection.
Floater terror missions are great for reaction fire training too. Because reapers suck. Kill ALL the floaters and run everyone up stairs or inside 1 tile doorway buildings. Pull out the laser pistols (or regular pistols). Ping away on 3-4 reapers from saftey (reapers can't go up stairs or through small entrances). Think of it as live target training. You'll be superb shots with good reactions in no time (actually, quite a lot of time, especially with basic pistols. It's like alien airsoft. Nobody gets hurt, everyone has some fun, and everyone wishes they had a real gun). Do not actively fire. Just get as many decent vision lanes as you can. Hopefully reaction fire happens. After turn 20, they can see you ANYWHERE. Make sure they can NOT fit through the doorways or go up the stairs (to the point of removing the stairs). Then just bunker down, keep pinging them, and hitting end turn. Takes forever, and you'll get somewhere between 0-6% more reaction stats off SOME people. Yaaayyyyy?
!/?
You can also stun the last few aliens on a level (leave one alive and kicking, or mission ends), take away their weapons, and let them run the gauntlet. Do it against any alien you stun really. But again, you lose a soldier or two who just sits there until the alien wakes up. And a live capture. It sort of feels contrived.
tl;dr. Don't rely on overwatch reaction firing. It sucks. The aliens are good at it. You are not. You play actively. They're playing UFO Defense. You die lots or waste tonnes of time otherwise. Use heaps of smoke. Aliens hate Xcom smoke. Always duck. It shows Earth who has the balls to teabag alien scum around here.