I have had experience with using both APEX and Mendix. On one of our projects we used both tools to come up with two solutions. From our experience we found that Mendix was a lot more adaptable and development speeds were much faster. This was because APEX is for just creating web front ends and not a database modeller. You have to make the changes to the database separate and then adjust all of the web pages individually. In our case this was about 180 pages that needed to be changed, so took forever. Another disadvantage is that you cannot create the microflows that Mendix offers. This really limits APEX and makes it very difficult to add business process/ customisation. Another thing the creating the style for the application in APEX was a nightmare whereas Mendix was a easy to use theme. Our company much preferred using Mendix to APEX. I would say APEX could be in the future a competitor but it still needs to grow. It is not a database modeller, just a Web application creator. Whereas Mendix excels at both modelling and web development. I hope this helps!
I have just had a look at the new features that APEX 4.0 offers and it has improved alot on previous versions. However the program is unable to create microflows yet! You can do similiar things but you have to program it yourself. New Features
I don't think you're going to get a completely unbiased opinion if you ask the people here ;)
However, I'm really interested in your findings, are you going to evaluate?
That's interesting. I had looked on the Oracle site but couldn't quite see what APEX does. However I just looked at [Look Inside] Beginning Oracle Application Express (Wrox Programmer to Programmer) on Amazon and it does give you the impression that if you want to add general purpose business logic you need to use PL/SQL.
I guess it's a bit like Access - effortless, supersonic speed at the simple things but come off the motorway and you are stuck in the slow moving traffic jam of third generation VBA coding.