Build Priority

Define "use priority". The feature itself, or the buttons on units?

Setting factory/nanos to low and mexes to high can be done automatically through Initial States but it's something you set once. In that way I don't use the priority buttons on units because they are already what I want most of the time.

I use the buttons in 2 main cases:
* sometimes I queue a giant line of solars in the back to make sure my energy smoothly increases over time. I set the con to Low so that the solar spam doesn't waste resources for more important stuff.
* crucial heavier stuff (Felon, Sniper, HLT) that I need ASAP gets High.

I don't use the buttons more because the main benefits lie in the above usage so bothering with it is probably a waste of time at my skill level.

Emergency-LLT or AA or similiar on 'high', when remember to click it.
My biggest fear is accidently forgeting some 'high' cons and then they use all resources, so I rarely change their settings. (Only the setting of the construction)

quote: or having mex or other structures set to always be high.

You mean there is setting to default all mexes or things to 'high'? I never bothered with looking for that tbh.

quote: A simple quantification of the principle would be when building both a mex and solar. Assuming 10 BP each and 10 metal/sec income, without priority they will both finish at around the 15 second mark. With priority, you can finish the mex first at around 7-8 seconds, and then finish the solar at 15.

I think in such situation I would not use priority. Cancel the solar, add it to queue of the mex-building con.
With priority toggle 2nd Con just stares at his project without doing anything.
Better if it sucks on trees or walks somewhere, like to the next mexspot. I think also more fun to manage buildpower by controlling units, not by switching switches.
But Commander I set to 'high' and use it for everything important. (Radar, defense, emergency-solars)

I use priority in early game and when I am in emergency such as: "I NEED THIS LLT NOE" or when I am estalling.

same as Orf.

btw, anyone else uses com/cons on repeat&priority when defender pushing?

1) You put your comm on repeat.
2) Your order a defender in the range of an enemy defender.
3) The first, second and maybe third defenders are blown to bits, but your comm is on repeat and keeps on trying.
4) Your defender is up.
5) Enemy defender goes kablooey

quote: With priority toggle 2nd Con just stares at his project without doing anything.

So now we know the origin of this!
http://commanderlookingatthings.tumblr.com/

I usually put my expanding cons on high priority. Expanding cons "waste" a lot of build power when moving so it is best that they get high priority when they actually are building stuff.
I usually put 1 or 2 cons on high priority making E on maps with a lot of metal. That ensures a steady increase in energy production.
Then I also micromanage priorities when I need some units to be built quickly. I always put nuke production as high priority because stalling as expensive buildpower as from a Silencer(nuke silo) really is a waste.
Shield morph, or area cloaker morph also gets high priority because such cheap morphs shouldn't really be slowed down by a stalling economy.

Steel_Blue
Low priority on your factory doesn't stop you building army unless you choose to make a lot of something else. It's almost always more efficient to either complete army first, or complete other project first.

The downside of putting your factory on repeat is that you will be less inclined to and slower at changing your unit composition. But that's better than excessing.
Low priority on fac is good on macro maps, on low metal maps you should be a bit more careful: only go for low prio if you know you don't need more units asap.

Flipstip
Low priority:
On small maps you just don't queue up as much non-unit stuff. Regardless of the priority on your fac, you can't afford it. Say we're on icyrun. It's as low eco as it gets, and every single non-unit decision is of utmost importance. When you make a mex you want it faster (since that's strictly more efficient than simultaneous unit+mex). If you want defenses you need it asap. If you need units you just don't make anything else. While intuitively it sounds sort of rigid to place units permanently on low priority, it's actually more flexible in this scenario (and all others I can think of). Low prio allows for more efficient production of everything else, while disallowing nothing default priority can do. It's a different way of thinking but I'm yet to see anything default priority can do better.

Repeat:
Repeat is more of a mixed bag. It makes it so that you never excess unless you forget infrastructure, which is great. But it disallows you from saving up metal 'lumps' for rapid construction of other things, like turrets or fac switches. I'd argue that your composition is equally malleable (though lazy players can use repeat as a substitute for composition). You set up your one default unit (for early game I choose a raider, for mid-game I choose an assault or skirm), then come back to individually make anything that isn't that unit. I make every worker, riot, aa, artillery, and scout this way, and most assaults and skirms this way. Essentially, worst case scenario you're having to manually select unit counts like you'd do otherwise.