Uber works OK in Riyadh, and I would recommend that. You can often wait a long time, though that's due largely to the horrible traffic in the city. Drivers often cancel rides en route to you. The cars are often dumpy, if you care about that, though so are taxis.

If you take a taxi, be prepared for difficult fare negotiations and also have your phone ready with the address/route unless you're going to a major landmark.