we used to manage nearly 120 databases(different OS,difer db versions), world wide using OEM. IT IS COOL. I never even had a unix login.
using OEM u can monitor databases, IAS, OID , forms server etc.
there are certain issues.
like certain versions (not very sure) as 9iAS V 9.0.2 heavily dependant on the included OEM.
and 9iAS 9.2 is not certified with oracle 9.2 db.
I was told that, Oracle 9iAS 9.0.2 work as charm with oracle DB 9.0.1.
good information is available with Oracle Enterprise Manager Readme Release 9.2 Production

1. You can create SUPERADMINISTRATOR's or REGULARADMINSTRATOR's.
2. Super Administrators automatically have full
   privileges for all objects in the system. 
3. Regular Administrators can have access to a subset
   of Console operations.
4. Target are listeners,databases,Nodes and HTTP
   servers.
5. So by assigning different target to dbadmins and
   appserveradmins (like just HTTP) we can segregate
   them.

something, i am not comfortable is,we can restrict access to some objects, but not restrict which services they can see!!.