I have one and find it practically useless. The thing is too small in circumference to properly cover a good area of an 8x10 let alone an 11x14 sheet of paper. I find that I am making my judgment on too small of an area of the print.
Probably whan I was just beginning to learn how to print it helped, but now I find it far easier just to do a step series on a single 8x10 sheet. I still fine tune in the next print or two to get the right grade but I am much more confident in where I am going. Also because I have done density/print grade testing on the four papers I commonly use at a few specfic enlarger head heights, I already know where abouts (aproximately) in time I should be printing to get my maximum black without excessive over exposure.
As a side note.... the time it took me to do the density/paper grade testing has proved to be worth it's weight in gold both in teaching me about paper grades as well as making it easier to select a starting print time.