I received a personal demonstration of the Web interface of OASsys (written in Oracle Application Express) from Jonathan Rhind of Zippy Zebra. I was impressed not only with their expert knowledge of Oracle Application Express, but also their competence in Web technologies and user interface design. Most Web applications are "okay"; this system was beautiful.
You can read the full details in the case study on the Zippy Zebra site. Please note that the system is targeted to support a user population of 30,000, all running on Oracle Real Application Clusters (Oracle RAC), with Oracle Database 11g Enterprise Edition.
And who said APEX is only good for Access or Excel replacement?