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.
Little outdated nowadays I have no experience with these old Symbians (or Symbian predecessors) so I can't judge it. I have only funny experience with 5800XM, C6-00 and a few other's. And i don't like it (today). When it was my "Device NO.1" i loved it. (playing Bounce... yeah ) but in comparsion with Maemo (and also Android or MeeGo) ... sadly i must say: ..Symbian is not a good OS..