I have recently started riding again and most of my bikes have been suffering from drive line issues; phantom gear changes, missing gears, basically just ruining the ride. Despite much fettling (and I have fettled successfully for quite a few years) these problems continued. I'd think I'd retuned the gears and in a few miles back to square one!
Anyway, I decided to bite the bullet and buy a new tool. I got
one of these and it's been a total eye-opener!! I never realised my derailleur hangers were so out of line. After about 15 minutes on each bike with this tool the bikes all seem to be shifting brilliantly.
Should've got one years ago! IMO there's no point in trying to fine tune your derailleur unless you've used one of these first!