He's had a very clear impact. Before Snowdon thinking that the NSA was collecting data on EVERYONE was a conspiracy theory. It was "crazy". Post-Snowdon it's a proven reality. Snowdon showed how far a clandestine agency with very little oversight can go. The answer, it controls EVERYTHING, it's been proven that not only have they spied on high up allied politicians (bad, but maybe excusable) they also spied on OUR OWN.
Snowdon showed us there's this agency, with a virtually unlimited budget that can control the politicians that control the world... and they are beholden to no one, the law is a rubber stamp.
Worse, even when it was demonstrated that they outstepped the boundaries of our laws... things have remained unchanged.
Snowdon showed us the truth, then left it up to us to do something about it. Unfortunately, no group of people has so far mustered up any change. That's our own failing.
Who Snowden is/was working for really is likely not of much importance. What matters is what impact he had, which at least to me, is not that clear.