During this year ODTUG Kaleidoscope'08 conference, almost everyone is talking and comparing APEX with ADF and ADF with APEX. It sounds quite funny for me and just a waste of time in most of the cases. Both products are good and both have areas where can be used. However, APEX is not a central and strategic tool in Oracle. JDeveloper/ADF is strategic combination and powers next generation of Oracle Applications.
Basically speaking, as Tom Kyte said during his "How Do You Know What You Know..." keynote on Sunday - "APEX is a database browser". APEX power is in rapid and departmental style applications, when user friendly database interface is needed.
JDeveloper/ADF is a different story, this technology allows to develop organization wide system and to solve so important nowadays integration problems.
I have heard, it happens in organizations with dominant PL/SQL expertise, that people suggest to develop complete enterprise systems only with APEX :-) In my personal opinion, both APEX and ADF are the best in their areas, just its very dangerous to think like this - "since I'm expert in this product, I can do everything only with this product".