Blog “The Eclectic Light Company” published excellent and very helpful review of DriveDx. Here are some quotes:

• DriveDx (by BinaryFruit) is fairly expensive compared with these other options, but is the most complete and extensive drive checking and monitoring tool available for OS X. What is more, it comes with uniquely helpful interpretive advice, which helps you work out what each of its gamut of figures means.
• DriveDx is one of the most important utilities available for Macs. Even if you only use a single built-in SSD, it provides important insights which could save your data from creeping drive failure, and will stop you from worrying about the insignificant.
• Buy it. Use it. It is worth as much as the data stored on your drives.

You can find full in-depth review here.