I have eight of them. Fairly nice effects if you add more details on top. There is a bit of a learning curve when you use different putties. Generally I use the following for each.
Green stuff/brown stuff: Vaseline
Milliput/Magic Sculp/Procrete: Talcum powder
Fymo/sculpy: Doesn't seem to need it.
The trick is to get a level surface on the putty. As mentioned the rings will definitely help. You can use one of their flat rolling pin or a regular baking pin to level the surface.
Here are a couple of examples.
On this image you can what happens on an uneven surface.
To address your concerns about the pattern repeating, it can. If you move and rotate the roller every time you roll a base this will alleviate that problem. I can say the service from them was top notch and I am happy with every product that I have purchased form their site.
Hope that helps.
Snitchy sends.