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I'd agree with you that RISC OS has a fantastic user interface. When it was released it was far ahead of its time, making Mac OS and Windows look hopelessly primitive in comparison. In my opinion it's still much more elegant and productive than Windows, Mac OS X, or any Linux GUI.
Having said that, the underlying OS really is showing its age. It was decent enough back when it was competing with Mac System 7 and Windows 3.1, and it's certainly lightning fast, but we're no longer using computers with 25Mhz CPUs and 1Mb RAM. It's usable, in the same way that Windows 98 or Mac OS 9 are still usable, but it's very much an OS of the past, not the future.