We are migrating four datawarehouse database to 9i (from 8.0.4) at the moment and we are going to use UNDO_MANAGEMENT=AUTO. So, I can't offer you any feedback abou tit yet, but I'm hoping it's going to be a large improvement over the traditional RBS segment method. I had a 10 GB tablespace for my old rollbacks in 8.0.4... I'm going to use that 10 GB as a UNDO tablespace now and see how it goes.