Conclusion:
Nexus has done a superb job of balancing not only great performance but also extreme silence with the XiR-3500 CPU cooler. Although we haven't tested a ton of CPU coolers in the past, this has certainly been the quietest cooler that has passed through my hands to date. I was impressed how at the same time it was still able to maintain such strong performance, especially in a case already built with low RPM fans with silence in mind.

Pros:
Silent operation
Doesn't loose too much performance for the silence it offers
Copper and aluminum versions available
Multi-platform

Cons:
AMD installation bracket wasn't the easiest to use (it worked though, but it took time to line up)
Non-replaceable fan