I have one, it is easy to read and takes a bit to set up correctly, I use a calculator. Saw needs to be in a vice that stays in the same plane as it is moved to see better, otherwise readings will be off the amount the saw tilted. I can only use mine on my tree monkey vice, the other ball socket stand I have needs to be used with a wheel to ensure it stays the same. Also, when going back to the project to check again, it needs to be re-zeroed, once you get used to this it can be done the first try if you do the math right.
Long and short of it, if you have a good saw vice, go for it, if you don't, either forget it, or get a new one from tree monkey, you'll get none better and you will wonder how you managed so long without it.