The Germans had since 2008 a local data retention law that made it an obligation for the ISP's and networkproviders to keep all data from all communications for 6 months. THere was a huge public outcry against this and the German High Court has now decided that the local law isn't a good interpretation/reflection of the European rules because it doesn't balance enough between the right of privacy for all and the need for the police to have the ability to monitor those that want to throw bombs at the heart of our democracy or are just plain criminals.
In Belgium we still don't have a local law and discussions in parliament have only started. Different versions and propositions are doing the rounds.
My personal take on it is that it is much better to invest heavily in specific infrastructure and human knowledge to give the police and intelligence services the exact info they need about a certain number of criminals and terrorist or espionage groups (even if it would be very easy for them to circumvent those intercepts) than to spend enormous amounts of money to keep everything about everybody in the hope that somewhere somehow something will come out of it (too late).
Some say that if they had those databases it would have been easier to track down the teacher serial killer. But that is not correct because his first attacks are already dated back to 2001. What makes the investigation goes forward now is systematic good policework and cooperation between the different services and departments.
Those investigations - like the one against the PKK going on since 3 years - need all the technology and material they need to prove their case or proof that they were wrong and there is no terrorism. On the international level this investment and coordination is even more important.
For example I think that as this terrorism trial is taking place in Belgium, security and intelligence services are closely trying to listen to the wires to see any hint of actions that could take place in the coming days or weeks. In a restricted but efficient Data Retention environment they would be able to spend less resources and make even more balanced opinions based on much more realtime information.