IBM AIX is a very powerful and extremely stable operating environment. It is well suited for applications that are business critical and cannot tolerate outages. It is best used to address large enterprise level application needs where stability and scalability are of paramount importance. IBM AIX is less useful for small enterprises.
AIX is robust, helps the systems administrator, is built to prevent easily made mistakes. If you are used to other variants of Unix (in particularly Linux) there is no steep learning curve to get started with AIX. You need to learn the intricacies of the operating system, but that is true for any new operating system. AIX has built-in tools for almost anything you want and has the AIX Toolbox (on the web) for tools that are not included with AIX by default, but can be installed. Installation of AIX is modular, you can select with components and features you want to have installed. Installation of additional components (and usually also removal of installed components) is easy and straight-forward.
Compared to other Unix-like operating systems, IBM AIX is the one with a solid roadmap into the future and is likely to stay around for a long time (it is over 30 years old and still being updated with new features and components all the time). Compared to many Linux variants, the AIX operating system is more robust, easier to manage and well-supported by its vendor.
