Am I the only one who finds flexbox incredibly complicated? It probably covers every possible scenario but doing some simple thing takes me 15 minutes of trial and error to get right.

I would say the opposite: After a small initial hump, flexbox is actually very simple for what it gives you, but it's ultimately fairly limited. It's not up to implementing every kind of grid layout cleanly, and it's not even in the same ballpark as constraint-based layouts.

I'm no CSS expert, but I personally find it incredibly easy and intuitive compared to the usual CSS alignment nonsense. Floats? Table display? Inline block?

Flexbox is the first CSS layout feature that makes me feel like I don't have to harness an army of hacks to get anything done.

There are still layouts it can't solve, for which I'm using Javascript to manually calculate the size of elements

There's a learning curve, but it isn't as fragile as the set if margin and position hacks that were standard before. Those old hacks are still used for supporting old browsers (IE8 and before), but flexbox simplifies things once you grasp it in applications.

I find it no more complicated than things such as figuring out things like clearfixes were in the beginning.

While you're right that the current versions of all browsers seem to support it now (though Safari still requires prefixing), it's pretty recent on some of these: http://caniuse.com/#search=flex. Most people still won't feel comfortable using it for probably another year or so, and this is just one element in the Flexbox specification; it doesn't necessarily imply the rest is well-supported or that implementations are reasonably consistent across browsers.

It is recent but I'm seeing >85% of visitors to non-technical sites using a browser which supports flexbox. At that point, using something like a CSS preprocessor to manage the prefixing / older spec support and deploying flexie for older versions of Internet Explorer starts looking like a very reasonable prospect. If your goal is simply to do vertical centering, align elements with consistent sizing, or have the ability to reorder elements in different contexts without having to change the DOM order, flexbox works well in all of the browsers listed at http://caniuse.com/#feat=flexbox.

That said, it is something of a travesty that one of the most popular reasons to dive down that particular rabbit hole in 2014 is to get real vertical centering: https://gist.github.com/acdha/a91a46706de02c37566a is still 11 lines for one of the most basic layout requirements.

OK, fair points, but if the OP can look back 8 years and apply that to now then I reserve the right to look forwards 8 years and do the same :)

So it only took 20 years...

...and it's more complicated and less capable.