When we meet with customers and explain to them that Oracle Application Express is great tool and that it can actually be used for their application development, the comment that we hear the most is : "Apex is made for small applications only ! ".
Why ? Well I think that is beacuse of the way that Apex was initially sold, an MS Access replacement (Maybe Oracle Sales also tend to push IAS a bit more...)
Having used APEX (Htmldb) since version 1.5 (April 2004), I can certify that Apex can compare with pretty much any other development software out there. It is scalable, it is easy to use and it is easy to install/upgrade.
During Openworld 2008, I will be part of a round table about SAAS and APEX. We currently support over 40000 user accounts and 20 different customers public web sites on a single APEX instance. Come see us, I will be more than glad to meet and discuss about APEX with you.
You still don't believe that APEX can be used for building complex applications ? Well, check out Tom Kyte blog post on the Oracle Application Express book, that should convince you.