Best of Breed
Mutator has literally been the only one than has exposed the bugs from Dr. Rose’s challenge for mutation analysis.
Flexible and Universal
The fact that TestLess has been extended to many languages and testing syles, including Adobe ColdFusion! 
Mutator and TestLess can handle diversity like no other tool (see item above), and have therefore been involved in several high-profile quality analyses with Ortasks’ How well is it tested? posts that report and expose the real quality of popular projects. Here are a few:
The fact that Mutator is free to use!
The video that started it all
Use in Context-Driven Testing
And finally, I’d like to emphasize a very important point that make Mutator and TestLess a success in software engineering management as a whole:
Tools that require more human engagement and control are preferable to those that require less. This is due to a syndrome called “automation complacency,” which is the tendency of human operators to lose their skills over time when using a tool that renders skill unnecessary under normal circumstances. In order to retain our wits, we humans must exercise them. Tools should be designed with that in mind, or else when the tool fails, the human operator will not be prepared to react.
For this very important reason, the use of Mutator and TestLess are perfect for context-driven testing!
 Via personal communication from customer