What is most valuable?
Query tuning, historical graphs, real time statistics on query usage, easy to find blocking or locking queries, easy to kill queries, easy to add new datafiles to tablespaces about to run out of space, easy to explore table structure
How has it helped my organization?
For Oracle databases we setup e-mail alerts through OEM. Setup several maintenance jobs to handle previously time consuming tasks such as automatically adding datafiles to a tablespace about to run out of space. Improved root cause analysis on database slowness issues
What needs improvement?
For databases and servers with high blocking issues, the monitoring and graphs in OEM frequently became so over loaded that they could not be used and would not load making OEM useless for troubleshooting certain production issues. When this would happen, I had to go back to using queries from the sqlplus command line.
For how long have I used the solution?
What do I think about the stability of the solution?
OEM agents on database servers frequently stopped collecting data and had to be restarted.
What do I think about the scalability of the solution?
Unknown, wasn't involved much with scalability but the Enterprise Manager web based site did get a little slower as we added thousands of servers.
How are customer service and technical support?
Oracle support is helpful and knowledgeable.
Which solution did I use previously and why did I switch?
Used Toad a little bit but didn't have a solution exactly like EM. Toad licensees were expensive and had to be installed on each user's machine while enterprise manager was web based and I'm sure Oracle gave the company special pricing since we already used so many of their products.
What other advice do I have?
Use as many features as possible but the more you rely on Enterprise Manager, the more you must remove and SPOF by having redundancy (RAC clusters or whatever failover solution you choose) and having a hot backup in a separate data center using data guard or some other replication technology so that if your data center goes down you can still bring up Enterprise Manager some where else.