AIX is probably the expensive UNIX variant I dislike the least. You don't really see any UNIXes (HPUX, AIX, Solaris, or any of the more-esoteric derivatives) outside of [i]very[/i] niche situations - HPC, scientific computing, some law-enforcement applications, high-frequency-trading, and the inevitable custom-built applications designed by some programmer who left the company 35 years ago and took the source code, debugging information, and troubleshooting guides with him. [laughs] 

The same goes for POWER-series devices - virtually every serious, commodity enterprise application runs on x86-64, whether you're talking about databases, SAN, IdP/SP, load balancers, VDI/VSI/VNI, financial systems, health care systems, or anything else. That said, I have worked, and do still work with them - the last system I had my hands on was AIX running on a POWER7 machine, providing a sensitive and mission-critical database. Running a pen-test (Although that job was so long-term it almost became a red-team exercise) against that system - and everyone who maintained it - [i]was[/i] a lot of fun.