This is a very solid strategy to use when up against a Terran opponent. First get out a Gateway, Cybernetics Core, Gateway, then a Robotics Facility. Get an early Zealot and a Sentry to Force Field your ramp if you're under early pressure. As soon as your Robotics Facility gets done, make an Observer. This will give you a great amount of intelligence about what your opponent is doing so you can react accordingly.
Once you have your Observer out move it through the attack paths that your opponent would take and over into his base to see what he's doing. He will not have any detection yet, and if he really wants to kill your Observer he will have to waste a Scan, which costs him roughly 260 minerals. By having your Observer in his base you'll know when he's moving out and can have the freedom to move around and take Xel'Naga watchtowers as you please.
Depending what you see with the Observer will be what route you take. If you see a lot of Barracks down and Marines coming out, throw down a Robotics Bay so you can get Colossi. When he notices you're going Colossi he's likely to make a lot of Marauders. You can counter this building a Twilight Council, researching Zealot leg speed and making lots of Zealots to deal with his Marauders.
If you see him heading fast to Tanks or Thors, simply build Immortals out of your Robotics Facility to support your Gateway units. It's important to know that your main army will be made out of your Gateways, so when you expand make sure you have Warp Gate Technology and a lot of Warp Gates to bring units out onto the battlefield. By having your Observer in the Terran base you can see what he's going for and then support your main army accordingly with your Robotics Facility and other buildings.
