It's hard to answer your question without any background information. Is the complexity justified if you just want a blog? No. Is the complexity justified if you're building a company intranet for 400+ people? Yes. Is it a good investment if you're looking to be a consultant? Absolutely! There's a lot of Plone work out there, and it pays much better than the average PHP job.

I'd encourage you to clarify what you're trying to build, and ask the Plone forums for advice. Plone has a very mature and friendly community — and will absolutely let you know if what you're trying to do is a poor fit for Plone. You can of course do whatever you want with Plone, but there are some areas where it's the best solution available, other areas where it'll be a lot of work to change it to do something else.

Some background:

The reason for the complexity of Plone at this point in time is that it's moving to a more modern architecture. It's bridging both the old and the new approach right now, which adds some complexity until the transition is mostly complete.

Plone is doing this to avoid leaving their customers behind by breaking backwards compatibility, which they take very seriously — unlike other systems I could mention (but won't ;).

You care about your data, the Plone community cares about their data — and we'd like you to be able to upgrade to the new and better versions even when we're transitioning to a new architecture. This is one of the Plone community's strengths, but there is of course a penalty to pay for modifying the plane while it's flying, and that's a bit of temporary, extra complexity.

Furthermore, Plone as a community has a strong focus on security (compare it to any other system on the vulnerabilities reported), and a very professional culture that values good architecture, testing and reusability.

As an example, consider the current version of Plone being developed (what will become 4.0):

It starts up 3-4 times faster than the current version.

It uses about 20% less memory than the current version.

There's a much, much easier types system in the works (Dexterity), which will reduce the complexity and speed up the system a lot, while keeping the same level of functionality

The code base is already 20% smaller than the current shipping version, and getting even smaller.

Early benchmarks of the new types system show a 5× speedup for content editing, and we haven't really started optimizing this part yet.

— Alexander Limi, Plone co-founder (and slightly biased ;)