I have been using Oracle Enterprise Manager since last 8 years of my Oracle DBA experience. Here are few of the many things that i love about OEM:
> Base Version of OEM is absolutely free given you already have licensed enterprise edition of Oracle database and tuning + diagnostic packs.
> It's POWERFUL. You can monitor database instances, listeners, hosts, storage (ASM), file systems, almost everything that you need to monitor and ensure your databases run smooth.
> Database administration is super easy when using OEM for lot of day to day activities like performance monitoring, diagnostics and alerts, user management etc. I can literally do all my DBA activities without having to login to individual database environments and without the hassle of maintaining all the SQL commands i need to achieve my tasks.
I love OEM but then there are few things, i don't appreciate about it.
First, It requires an intense knowledge of OEM both for installation and configuration. I don't mind it, most DBA's won't. But for someone who is just starting up with OEM, it can be super overwhelming.
Second, Given that OEM runs on top of Oracle WebLogic Server, the DBA also needs to have knowledge of WebLogic Server Administration. If Oracle can make it a little more seamless where DBA doesn't has to worry about WebLogic portion of the stack, it would be awesome.
We are using Oracle Enterprise Manager for monitoring and administering all are databases. Before OEM, we had to create hundreds of monitoring scripts to monitor all our databases for different scenarios of failure. This was tedious and not very reliable.
With OEM, monitoring and administration has become much more reliable and easy.