I haven't seen a single PF in the game. Does anyone build them?
There are a number of times when a Planetary Fortress is useful, but before we get to that let's look at what a PF is:
- 0 Additional Supply
- 40 AoE damage (20 dps)
The biggest cost of a Planetary Fortress is the lack of building an Orbital Command. If OC wasn't so useful, there is little reason not to build the PF. It has heavy HP and Splash Damage, and doesn't add toward your supply max.
So when does the ability to have an extra Siege Tank worth of damage out weigh the benefit of Orbital Command? Well there is one situation where you have Command Centers in vulnerable positions that may not have quick reinforcements: Expansions. Actually if you watch a lot of high level matches, you'll start to notice that a 3rd or 4th expansion often becomes a Planetary Fortress. Since you're expanding past your natural by the 3rd, you often don't have your army in convenient position. As a result, Planetary Fortress helps to delay attackers and often repel harassment. Ever try to kill a PF with just Zerglings? it ain't pretty.
So next time you're thinking: do I really need a Planetary Fortress? ask yourself this: Do you really need that 3rd expansion to stay alive?
They are quite usefull and if you watch some highlevel replays they are used quite often.
The best use is at hard to defend expansions where it can delay the enemy until your units arrive which can allow you to take more expansions than the enemy (but it's no replacement for troops).
Some people also place them at choke points where they add a lot of defense due to the high HP and quite good damage, but they are quite expensive, so you should have an expansion advantage before even thinking about using a command center for this instead for another expansion.
There is also the somewhat unorthodox cheese strategy "Planetary fortress rush". I would not recommend this because..well, it's just not that good to be honest :) I've never seen it used against me and I've never tried it myself.
But it's a fun idea! http://wiki.teamliquid.net/starcraft2/Planetary_Fortress_Rush
To add to tzenes' great points:
I've seen high-level plays where they build extra Planetary Fortresses in Yellow Mine expansions to fend off attacks more effectively.
Late game, building Planetary Fortresses near a choke point will help increase your defenses.
I successfully used PT on an expansion site yesterday. Playing agains a Terran and saw he was going to rush with MMM. The PT successfully blasted a good bit of his forces and held it's ground until reinforcements arrived. After that experience I am now a big fan of PT on exposed/vulnerable expansions