It is a massive success, primarily because by the time the Montreal Protocol was fully ratified it was more profitable to not use CFCs.
However, speaking as someone who lives at the bottom of the world in the country with the highest melanoma risk in the world we didn’t actually fix it. We stopped the holes in the ozone layer growing and saw some recovery, with the hole over the Northern Hemisphere predicted to close by 2030-ish and ours by 2060-ish, but it’s nowhere near fixed.
And since about 2013 we’ve seen a massive increase in CFC emissions again, so the Southern Hemisphere hole is probably pushing out to 2070-ish. Not that any scientific research has definitively stated that yet, it’s mostly non-committal. The majority of these new emissions have been traced to countries that didn’t have to get rid of those specific CFCs until 2010, so it’s a good indicator that those countries may view the Montreal Protocol differently in the new millennium than they did in the 80s. Or it indicates that it’s taking them longer to cease usage than predicted. Hard to tell really.
So to say “It’s fixed!” is a little hopeful. The problem still exists, and effects are still being felt, but there’s nothing you or I can do - hence the common narrative, especially in the North, that all the hard work was done in the 80s and we’re good now.
I suppose I question if the Montreal Protocol and associated social movement was a success if we didn’t completely stop the damage (sorry if you live underneath a hole and have a stupid skin cancer rate as a result) and we’re backsliding (Oh you were a developing country when this was signed, keep using CFCs til 2010, but we won’t enforce penalties if you’re still doing it in 2013). It was incredibly impressive to get the buy in that the Protocol got, especially given the other stuff going on in the world at the time.
But on reflection I’d hesitate to call the thing a success.