I have a mountain ash myself, currently growing in the ground. It seems to be very apically dominant and all shoots want to grow straight upwards making it somewhat difficult to style. The leaves are very nice though. Hopefully I will get flowers and berries one day. Here in Sweden we say that if the mountain ash has plenty of berries in the autumn, the winter will be very cold. This is probably not true in California though
Here are some useful links:
http://www.bonsai4me.com/SpeciesGuide/Sorbus.html
http://www.bonsai-wire.com/articles3.htm