The robust functionality and the continuous upgrades. I've been frustrated in the past that axure didn't allow me to select which corners I wanted rounded or which sides needed borders, but those have been added. It's excellent for prototyping and allows me to do basically anything I would need to test.

Flexibility and intuitive task flows make it pleasure to use.

Axure allows for nearly everything a UX Designer could desire. The capability is extremely extensive, allowing for wireframes and prototypes with extreme detail. The software utilizes intuitive thinking and UX Design to provide the UX Designer themselves with the best tools to wireframe and prototype.