RPX recently released their 2017 in Review report which showed the 10 most prolific NPEs of 2017 and 2016.
RPX graphic
Unified has independently challenged every single prolific NPE at least once. (Unified filings). Studies have found, consistent with Unified's experience and research, that prolific plaintiffs are very often NPEs asserting lower quality patents. This creates a vicious cycle of litigation and settlements which depresses the value of truly innovative patents and unnecessarily saps the resources of all companies. That is why Unified never pays NPEs.
RPX, another 3rd party, has paid NPEs hundreds of millions according to their financial statements and independently filed against only 3 of them. (RPX filings).
You can see a full comparison here:
https://www.unifiedpatents.com/success