I have been using RISC OS for about 31 years, and it is still better suited to my use than any other OS that I have used. The best font rendering of any OS, true drag and drop (not the fake nock offs of win/mac/x) that is truely pervasive, easy and intuitive GUI (called WIMP for Windows, Icons, Menus, and Pointer), and easy to write programs for.
On the pros:
-RISC OS is reasonable written, taking little memory/CPU time to do its tasks. With large parts still written in ARM Assembly Language.
-RISC OS is logical enough for a person to be able to understand completely in every detail.
-RISC OS has applications for graphics, video editing, and similar feilds beyond the abilities of others still to this day.
-RISC OS is very intuitive and makes being productive very easy.
-BBC BASIC V (AKA ARM BASIC) is part of the OS, including the inbuilt assembler.
-It is actually reasonable to watch streaming video (like Youtube) on RISC OS, with out the slow down you get on other OS's.
-Applications generally do not waste memory or CPU time for no reason.
On the Cons:
-There is not yet very good support for SMP, HW Graphics Acceleration, or 802.11 WiFi networking.
-Above is the only con I know of.