Once you understand how the software works, it becomes fairly intuitive to figure out how to accomplish what you need to do.
I like the fact that it is web-based, and depending on your organization's security setup, you can access it from anywhere.
It is a fairly robust system, and while there may be some things that the system just will not do or allow, those things are few and far between. With help from a good consultant, you can achieve almost any setup you desire.
Java can be a real pain. We set up our users' systems with the right version of Java, the right compatibility mode settings, etc., when they are given their computer. However, when there is an update to Java or a patch to Oracle, it can be a real mess trying to let users know how to update those setting and still be able to access our Oracle instance.
The reporting from within Oracle is disappointing-- often ugly and in basic ASCI text format, rather than a spreadsheet (or even a PDF).
Reports are also limited in scope, so organizations that need robust reporting capabilities will likely need to purchase additional reporting software.
I highly recommend working with a consultant group to help with your implementation, but more importantly, to also to have those consultants work closely with an internal team that will become the internal consultants for your organization. The maintenance and ongoing evolution of the software is extremely important-- you will need those individuals to be the subject matter experts on the different modules of the system once go-live is over and the consultants have moved on to another project.
There have been several process improvements across our organization due to the implementation of Oracle. Many of our operating companies previously used software that had much looser control on inventory, invoicing, accounting, etc. (Software such as QuickBooks, MYOB, and more)
While there is a little pain for the users in moving to a system with tighter controls and less forgiving finance and accounting modules, it ends up forcing users to really follow best practices, rather than being able to manipulate an ERP system with weak restrictions to work around what are essentially "broken" processes.