Apparently the UK has been participating too:http://www.guardian.co.uk/technology/2013/jun/07/prism-gchq-access-covert-operation
It sounds similar to the joint-US, Australia, Canada, New Zealand and UK ECHELON system, which has been going since the '60s and has been public knowledge for a long time. Its purpose is the global interception of communication bearers including satellite transmission, public switched telephone networks (which once carried most Internet traffic) and microwave links. Its main UK satellite ground station is located a few miles away from me.
I've always assumed my internet and email activities were monitored to some extent, not individually, but in a very general way for the purposes of counter-terrorism. Does it bother me? Not really. It certainly would if I was living in, say, China or Russia and it was those countries' governments doing the monitoring.