The Photon Cannon. 20 normal damage vs air or ground. Better range than most air or ground units. 100 shields 100 hp. Some people think they are the savior to Protoss, some think they are worthless. I agree with the 1st, but not the way you think.

For 150 minerals you can take up a 2x2 space that will do 20 dmg to any enemy unit. They are stronger than any protoss ground except archons (light or dark), but they're stuck in the ground and dont move. Tanks, nukes, reavers, disruption web, guardians, dark swarm, or just running around em. Counters that each race has. Still, they are sturdy to an extent and do pretty darn good damage.

Yes, they are very useful if used right. I am focusing on the sturdy part and that (unlike a pylon or whatever) they are things that will make units auto-target them (on an attack-move units will still shoot em even if they're getting shot at, themselves). Reavers, High templars, zealots, dragoons, dark templars, corsairs. All the support that your cannons need for an near perfect defense. As hydras/lings/dragoons/zealots/marines/firebats start attacking the cannons (you should have about 6) hurry and get your hi templars storming them. or disruption web them. (them = enemy units) Or get your force of zealots/dragoons/reavers attacking. So right now we have soma cannons getting attacked by units that are being psi stormed or are under a disruption web and getting the sh** beat out of them by reavers dragoons or zealots.

If you're getting dropped, have observers to check when and where, and corsairs on the ready to kill the droppers (just watch out for fake drops (hallucinations)).

CreepColony's Opinion: Many newbies think building tons of cannons will save their pompis. But it doesn't :(