The old EPOC UI is alive and well and living in Southwark
Believe it or not, internally many of the Symbian developers still use the old EPOC UI, tarted up a bit and with support for new Symbian OS features like Bluetooth and such. It's used during bringup and in OS and system infractructure testing, since the Symbian OS is supposed to be UI agnostic.
EPOC was initially released to non-Psion vendors at version 4, with this Eikon shell that we all fondly remember.
During the SymbianOS 6 timeframe, two UIs were designed called Quartz and Crystal. Quartz evolved into UIQ (that's what the name means) and Crystal turned into Series 80. Since Symbian itself had to stay agnostic, the old Eikon UI was kept around for debugging and testing purposes.
Then Symbian prototyped a bunch more UI layers, Sapphire, Ruby and Emerald, which nobody seemed to want, but Nokia took Sapphire and turned it into Series 60 (mostly by mangling it til it looked like Series 40 enough for their users to use without having to think too much about it.)
And then, in the SymbianOS 7 timeframe, Nokia made another UI- Series 90- and did almost nothing with it, and then the Japanese cellphone companies made yet another one- FOMA.
So, yeah. There are a lot of user interface layers on SymbianOS. It's probably the only user-facing OS that really has more than two actively used complete UI layers. Of course, all the infrastructure (stuff like messaging, the POSIX layer, hardware drivers, filesystems, signalling and network stacks, and even higher level stuff like crypto/cert management, image manipulation and email handling) is shared between all of these user interface systems.
At least 90% of the buggy, slow code is in the vendor-supplied code. SymbianOS itself is fast and efficient- booting to the text shell takes under ten seconds, even on a good old ARM Integrator. Booting to TechShell takes a little longer but it's still considerably faster than even the relatively snappy N95's Series 60 boot.
Disclosure: I was one of the Symbian EKA2 kernel coders. I like SymbianOS a lot, even though these days I work on firmware for machines that run Linux .