About ten years ago, I went through this process with as many data brokers and reporting agencies as possible. I printed documents, made accounts, mailed letters, and jumped through every hoop they set up. Some of the companies only recognized my request for a term of five years, while some were permanent.
I don’t get random credit card offers in the mail any more. However, since I started using an email alias service a few years ago, I’ve found my data making its way back to the data brokers. I’m not in the EU, so I don’t think I have much recourse for it. Even so, I think it’s worth it to jump through all the hoops. Maybe one day it’ll feel more meaningful.
tl;dr: It was cumbersome to do it, and other than a reduction in snail mail spam, I haven’t seen much benefit from it. All the same, I’d do it again.