Data Migration from Informix database to oracle database

Hi experts,
I would like someone to help me with migration that I must execute soon. I will migrate an older Informix database to a newer Oracle 11g database. This is my first big migration experience and I have some questions.
First, I would like to know if Oracle Migration workbench already does what Oracle says and migrates easily or should I expect problems with its usage? My second question is, if I must do the migration manually, which steps do you recommend? I really need any help, suggestions and advice on this issue.
Thanks.
