I liked AIX better than HP-UX. But not as well as OSF/1 (DEC) errr Tru64 (Compaq).

AIX has a lot of features that makes it very easy to install system patches and run with them still not yet committed, able to be removed/backed out all on the fly. Also, has a lot of advanced features that make things work easier for installs and volume management.

Since Oracle purchased KSlice... that feature is now gone for Linux, except for "Unbreakable" Linux (Oracle's RHEL clone).