I cannot say I've used that one, but one of the professors at my University, Mississippi State University, wrote their own embedded operating system for the PIC24 Microcontroller.
It's called ESOS, for Embedded Systems Operating Systems, which was designed to be used for our Embedded Systems class, but can be used outside of school, as the source code is available.
http://www.ece.msstate.edu/courses/e.../_e_s_o_s.html
It was primary developed by Dr. J.W. Bruce, but the source and ideas used for the source have been pulled from multiple sources. Ahem... ALL THE SOURCES!!!
Anyway, it's another consideration to take for a µC OS for non-RT development.