How can I effectively use Tempests?

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:

• What's the most effective way to utilize the Tempest's long range?
• How does the effectiveness of Tempests change from PvP, PvT, and PvZ?