As per you the '323 when compared with '320 is cheaper, has a Vcc range of 2.7 to 5.5V, and can do all that a '320 can do EXCEPT run at anything more than 18MHz.

Speed and interface issues will be the main reasons why people dont migrate enmass to the '323. When migrating to lower voltage versions just doubly check to see,

* If all pins that will be exposed to 5V can tolerate that voltage when the Vcc to the MCU is at 3V.

* Availability of peripheral chips that are happy with 3V.

* Noise immunity of the 3V setup as compared to the 5V setup.

These are, in my opinion the starters. There is more to it - and if power consumption is an issue then the 3V stuff could be more attractive.

Raghu