Others have given my thoughts on these things. Good for when you've got absolutely no idea of likely exposure and suits the image in that the pie slices all cover important (equal) areas.
One hint that I learnt well after I'd stopped using the thing was the minute expose suggested (on the Kodak one) can be halved (etc) to suit your enlarger brightness then divide the selected exposure by the factor that you used less than the minute. eg say you exposed for 30secs and the 14sec segment looked good, then divide 14 by 2 (60 / 30) to get the 7sec time required. Easier in practice than trying to describe it but you might need a calculator!
Once I got my film exposure/developing a lot more consistant, I found it wasn't much use as I knew my enlarger exposure was going to be within a small window and the steps between 'slices' is too big. The good old test strip works for me.