Har-Tru that is properly maintained slides great! It helps if it is watered from underneath, but the most important thing is that the clay is replaced regularly. Over time, the grains get worn down, and it clumps together, making it harder to slide. Our local courts are in excellent shape, and I can slide 8-10 feet if I need to.
Maybe it's a regional thing, but here in Georgia, there are lots of public clay (Har-Tru) courts. Columbus, GA has a huge facility with a zillion of them. I live in a small town, and we are lucky to have a great public facility with 6 clay courts, and we're getting 4 more later this year.
With all of the green clay in the southeast, I don't know why American pros aren't better on it. I think it must be the way they are coached, or maybe they all attend academies with hardcourts. High School tennis is always played on hardcourts.
I'm a huge Har-Tru fan. I hate to play on anything else.