I'm not sure how well they work in practice in individual member countries. I recall some ISP guys complaining about how this required Denmark to essentially save all netflow data which would be a huge data storage requirement, but maybe it got relaxed. I can't say I exactly have heard of it being the base of any arrests.
Germany's Federal Constitutional Court invalidated a law that required ISPs to store the internet history and phone calls (who called whom and location they were made from in case of cell phones) for 6 months.
Afaik there are constitutional issues in several countries with the data retention laws the EU tries to enforce.
I'm not sure how well they work in practice in individual member countries. I recall some ISP guys complaining about how this required Denmark to essentially save all netflow data which would be a huge data storage requirement, but maybe it got relaxed. I can't say I exactly have heard of it being the base of any arrests.