I have succesfully used it to get my data removed from services where that would be impossible in the past (most notably, everyone who sends me emails that they feel I desire). There are more examples (like companies that finally have to think about what they're collecting and how), but on a personal level I think that's the most tangible and best one.

If people actually implemented a cookie banner that was compliant, I would like that they are there. Because that means I could filter them away and I could be safe in the knowledge there were no tracking cookies. As it stands, you can see a lot of angry people making things that sound acceptable in the US (by continuing to browse, you agree to [...]) but is actually just not compliant. That's a sad state of affairs, but it's not the fault of the GDPR in my view.