I'm not a very experienced player in Starcraft, but I've noticed that the new Tempest unit seems a little disappointing, despite its capabilities. This made me wonder if I'm using it incorrectly, and led me to a few questions:
First and foremost, you need vision to be able to maximize the range of your tempest. Oracle's revelation (like grubby does in the link) or an observer will provide you with this. Tempest are pretty expensive, take a lot of supply and are pretty late tech. You do not want them to be alone. You have to view them as flying siege tank. The tempest are pretty fast for how big and strong they are, but their slow rate of fire and low damage vs non-massive air will make them weak vs smaller units like viking and corruptors.
So, like I said before, use them like siege tank, literally. You can have 4-5 hover over your army, get vision of the location you want to attack with an obs or revelation and start shooting your target. If your opponent wants to engage the tempest, he will have to engage your army AND the tempest. Just like with Terrans and siege tanks, as long as your tempests are not out of position, winning the engagement will be hard for your opponent.
Situations they will help you in:
PvT
Terran is going mech, you can shoot the tanks/Thor from afar.
Killing that really annoying PF and forcing the engagement out of the range of said PF
Against siege lines, no longer do we have to send Zealots from all direction and hope for an engagement that will not be too bad for us, we can force it now.
PvZ
Broodlords army, you can shoot the BL from afar
Snipe a hatchery for a favorable position
Snipe ultras or Swarm Hosts
PvP
Colossus sniping (not sure if they have the bonus damage since collossus are both ground and air
Map Control
Momma ship snipe
The main weakness of the Tempest is their lack of splash damage, so your supporting army should have some colossi or Templars since a protoss deathball without splash isn't that hard to kill.
These are the situations I can think of, as I see more uses of them in tournaments, more situations might come where they are useful (pretty rare to see them because of how far in the tech three they are and they don't have splash damage).
EDIT: People asked for a game:
put the video at 1h13 mins for the Grubby vs stephano game
The most effective way to use the Tempest long range would be against a Zerg. If you can do a tempest rush of 2-3 tempest and hide them above a ledge, or over empty space, the Zerg will not have a unit that can attack them. They may have a couple of Mutas but Tempest can make short work of them.
If you can harass the mineral line and snipe any queens, you can effectively take care of Zerg using a tempest. I know that was a strategy done against me which worked quite well during the beta.
(Diamond / Master player) Allover - I would recommend using the Tempest as a "late-game" unit, since it is expensive. Going for Tempest on one, maybe even two bases is not good.
The all over role of the Tempest is air support to your main army, late-mid and late game.
A mass of Tempest isn't that good.
So to use and abuse the range of the Tempest, can be when you want to punch a hole in your opponents front line, even sniping buildings. Also for long range attacks alongside your main army.
But be wary of Vikings if you also have Colossi, the T will have a lot of them. Same thing for Zerg, be wary for Corrupters.
It's still equally as effective all over. It really depends on what your opponent is going for!
