Ha, I couldn't work out what _isn't_ a good UX about them until you posted this. It's worth noting a couple of extra things about them too, while taking up much less space than a slider they operate in the same way - this often means you can get a lot more range / precision than if you'd tried to fit it in a slider as your touch can slide way outside the bounds of the control itself. The other is that there's a convention in touch-interface audio apps that double tapping the dial will return it to its default state, whatever value that might be. This arguably could be done with a slider, but I've not come across that on any of the apps I use.