I've had it since May 2016. When I got it, I ordered 3 extra grinding rings, 2 different sizes (1/2" and 5/8" FIRE) and an extra in the main one I use (5/8"). The 1/2" I got for my son, the 5/8" for myself, and the FIRE to give it a try. Initially I was sharpening at my usual intervals, roughly every week or 5-6 hours of ice time. I've alternated between just doing a couple passes every couple of hours of ice, to waiting a week and doing a normal sharpening (4 passes). As to the number of rings I've used, I'm on the second round of rings in each size (1/2" and 5/8"). I don't use the FIRE often, I have a second set of blades that I sharpen with that, but just don't use them often. And I may switch my son to 5/8", we tried his last sharpening in that and he liked it.
No issues at all. Easy to use, and we've liked the sharpenings we've gotten with it. Based on the manual, I've probably got to change the air filter soon, but haven't had the warning light turn on yet. I will probably order one with my next ring order just to make sure I have it.