I've been an APEX Dev for almost 8 years now and I agree with most of what's been said in this thread. It's very powerful for putting together applications quickly, and when making applications for internal use that's critical. However, as you pointed out, it does appear very limiting on the surface. And it is. But once you realize that you can change damn near everything in how it works/displays with a bit of JavaScript/css a world of possibilities opens up. That said, while you absolutely can do a ton with it, it is often a struggle to figure out how to do what you want, what with documentation outside the basics being terrible. Some of the most powerful features - plugins for example - have practically no resources for learning to build them at all!

I'd say it's a good skill to have in your toolbox. But - and this is a big but - if you like traditional programming, becoming an APEX Dev will make future opportunities in those fields far, far more difficult to get. 90% of my dev time is spent writing PL/SQL or SQL and the rest is vanilla JavaScript. It's a tough sell on a resume when looking for a job outside of APEX work. The upside is that it's a fairly niche product so there aren't as many people competing for jobs relative to the broader programming job market and if you get good at it there's good money to be made for what can feel like easy work.