Of course, there are plenty of limitations in Uber's Pittsburgh pilot. Only an unspecified "small number" of vehicles are being deployed to customers, and only on certain roads and routes that the company has already mapped out and tested extensively. Uber told me that those routes are some of the most popular roads being taken by its normal drivers, so it's not like these are unpopulated back roads, but you can't take an autonomous Uber anywhere you want.
There's also a safety driver behind the wheel waiting to take over the car at a moment's notice. These handy humans can correct the car if it ever gets confused about what to do next, and they can also drive you off of the currently available autonomous routes to get you exactly to your destination. But if you're going far outside the range of where these cars can operate, you'll just get a normal Uber X instead; there doesn't appear to be any way to specifically request a self-driving car, so you'll just need to keep your fingers crossed. Hailing the car itself is pretty straightforward: The Uber app will show you if you're getting a self-driving car (should you want to cancel the ride or celebrate, depending on how you feel about autonomous vehicles).
The cars that Uber is deploying are Ford Fusions with a huge array of sensors on top, with more hidden around the vehicle. The main unit up top is a 360-degree LIDAR unit that shoots out some 1 million laser beams per second to make a three-dimensional scan of the world around the car. Of course, those scans aren't in color, so there's another camera right below it that recognizes colors so the car can stop at a red light. There's also a large front-facing camera array that looks for vehicles, pedestrians, lights, signs and so on.
Those are just a few of the sensors blanketing the car, and there's also onboard computer and storage units for processing data in real time. The whole thing looks pretty elaborate right now, but Uber also showed me a Volvo with a much smaller and more discreet roof unit. An Uber representative called the Ford's unit the "desktop" version and the Volvo's a "laptop," while promising we'd see the "smartphone-size" version before long.