I use mine only occasionally - when I am cutting other than 90 degree angles. I used to use it for 90's before I bought my Swanson Speed Square (see that review), but it does tend to slip out of adjustment if it is bumped around. The fixed 90 on the Swanson is always there.
This guide does have plenty of good uses if you are careful with it. I always use a "pattern" piece when laying out rafters. I check the angle of this jig on the pattern from time to time to make sure it is still set correctly. The wing nut will keep it adjusted pretty well, but it is tightened against smooth surfaces, which are necessary for infinite adjustment. The "pointer" is adjustable with a small screw and will stay put pretty well. It has a protractor style degree scale. I check it with my Swanson occasionally.
I may Modify mine someday by putting on a larger, jig-style wingnut that will allow greater tightening force, making it less necessary to check the setup repeatedly. I might also try gluing some sandpaper to one side to provide more friction in the screw area.