Is a Sensor Tower useful in Starcraft 2?

Solution 1:

The point of building the sensor tower is to sense enemy positions in fog-of-war, and highlight them in the main view and on the minimap. This allows a defending player to cover multiple potential attack routes and, when an attack is spotted, rally their forces and ready their defenses to respond to the incoming attack. It also serves to cover "unexpected" attack routes, such as air or a medivac drop coming from behind your mineral line.

(more of a pretty image than an effective use of a sensor tower.)

In most matchups, the enemy will already know where you are (eg on a 1v1 map), or can find out with little effort by sending an expendable unit around the map - so the outline only betrays the fact that you've spent 100 gas on a building, and that you've got an engineering bay. That's all. The sensor tower's outline on the minimap shows up to enemies primarily to inform them that if their units go inside this area, they will be highlighted. The enemy can use this information to either avoid entering the sensor tower's area until the last second before an attack, or to deliberately "show" forces to the defending player to scare them from attacking, and allow expansion.

If nothing else, the sensor tower can act as a deterrent from surprise attacks and harassment, because the enemy knows that you'll know they're coming. Players tend to stack up their units just outside of sensor tower range without even thinking about it, and a comsat scan just outside can reveal the composition of the enemy army.

Looking at the movement pattern of the red dots can also give away the composition of the enemy army, if you're familiar with how the units move, and at what speeds; a flock of mutalisks will fly in a very close formation and spread out quickly when stopped, while a bunch of reapers stop at every ledge for a moment as they jump.

Perhaps the most effective use of the sensor tower would be in deterring void ray / mutalisk / reaper sneak attacks; seeing a few units move towards your buildings over impassable terrain gives the game away, and allows you to have marines in position long before the voids are there to ruin your day. Another effective use is grabbing a sensor tower and some vikings, and putting the hard word on some nearby lurking overlords, robbing your enemy of precious vision and supply.

If you want to read a bunch of people talking about the benefits and drawbacks of the sensor tower, read the Sensor Tower thread on the TeamLiquid forums.

Solution 2:

Build it after you have been scouted. At that point they will know where you are anyway. A Sensor Tower will show you incoming enemy forces in a huge radius on your minimap and in the field of battle (even outside of your visible range--they appear as (!)).

Early warning of incoming forces is critical.