In my opinion, without Enterprise Manager no DBA can do their job. There are better tools out there, but they only complement OEM because it may not do the whole job. OEM, though, is very intuitive when it comes to performance tuning, metrics, and monitoring.
Improvements to My Organization:
Without it, it's like driving a car without seeing with your eyes. Without OEM, I can't troubleshoot any of the issues that arise. For the Automatic Database Diagnostic Monitor, port reports, memory advisors depending on load, manual memory management and other database analyses that SGAs and PGAs expect -- these are all things that can only be provided by OEM.
Room for Improvement:
It's quite user-friendly when integrating, but when propagating user access and agents, it's not so intuitive. I'm using 184.108.40.206, I believe, and there seems to be a log of bugs that haven't been worked out.
The issues with deployment in 220.127.116.11 have to do with user access and agents. Other than that, it deploys fine.
It is buggy in certain process, but every Oracle product has bugs, although unless you really hit it you would never really come to know about it. For example, one of the Java processes within OEM chews up the entire CPU. This is an obvious bug, but there is a patch that can be applied to correct the issue.
It scales with out issues for us.
Deploying agent is not easy, but the rest of setting up is fairly easy.
We implemented it ourselves with our in-house team.
You need OEM if you are having Oracle databases. OEM is very expensive, but it does some nice things like memory advisors and ADDM reports. There are products that complement OEM, but they can't replace it.