I mean OS as OS. I'm not Symbian developer, so I don't know how it realy looks "inside". I was Symbian user for long time, and what i know perfectly is the "stability" of it :P
However GUI is a part of OS - if you can't choose another one - so when you don't like the GUI, you dont like the whole OS.
For example multitasking in S60. Unusable.