I've used the new laws twice now to close online accounts with companies that were uncooperative or too 'clingy' (looking at you, OVH and Microsoft). Much easier to send a registered letter than waiting on hold or searching for an online option that was deliberately made hard to find, or which may not exist at all.
I have also used it to stop unwanted postal ads from local companies. I get to find out how they obtained my info, and also stop some junk mail.
For the sibling comments mentioning the GDPR popups / cookie notices, why not add a blocklist for these to your adblocker? At this point adblockers should be considered basic security software, like a firewall or antivirus. These lists exist are are pretty comprehensive.
As an American living in Europe I think it's a great law and I wish there was something comparable to protect my friends and family stateside. And as someone who administers a fair amount of business and client data, I do not find the law inconvenient to comply with. I am very pro-privacy and protective of user data, and I didn't have to make any major adjustments.