A very good article about migrating AIX from version 5.3 to 6.1 can be found on the following page of IBM developerWorks:
http://www.ibm.com/developerworks/aix/library/au-migrate_nimadm/index.html?ca=drs
For a smooth nimadm process, make sure that you clean up as much filesets of your server as possible (get rid of the things you no longer need). The more filesets that need to be migrated, the longer the process will take. Also make sure that openssl/openssh is up-to-date on the server to be migrated; this is likely to break when you have old versions installed.
Very useful is also a gigabit Ethernet connection between the NIM server and the server to be upgraded, as the nimadm process copies over the client rootvg to the NIM server and back.
The log file for a nimadm process can be found on the NIM server in /var/adm/ras/alt_mig.