I regret going FX. Especially in tricky situations where I can shoot at f/4 in non-existent light with high ISO and still get amazing images.
Wow. I am still looking for my first image that I could call "amazing".
I'm still waiting for "the truth"
OK. I will try to explain. There is, of course, an undisputed difference in IQ between a crop and FF. This difference, though, is purely technical. Less noise at high ISO etc, we all know about it. But all these differences mean nothing, absolutely nothing unless they become meaningful for a particular image(s). If we fail here, what we will get, at best (see above), is an "awesome looking image that is not good".
It depends on a genre, of course. With astro photography, which is quite technical, these FF qualities, probably, do shine through, excuse my pun. With many other genres it is not that obvious. One has to be quite advanced in his/her photography for these differences to become MEANINGFUL for their images. I often see images taken with a FF camera that would be exactly as good/bad/average as if taken with a crop.
Maybe it is just me, but I tend to look for an artistic intent/aesthetic side first and foremost, and if it is not there, I do not care about technical stuff like colour gradation or high ISO noise or any other advantages offered by FF. More than that, sometimes I see ( DISCLAIMER: NOT ON THIS FORUM

) a "test" photo by some new proud FF camera owner with words "Just tried my new full frame camera - WOW! I am so impressed!". I honestly try to be impressed as well, but for some reason fail. The image looks no better than if it was taken with a crop. I guess a computer screen is not the best tool to evaluate an image quality? Or maybe there is another reason?
The way you work with the light, for example, is SO much more important for the quality on your image than the difference between FX and DX, you can not even compare it. If your work sucks here, the difference between FX and DX becomes negligible. It is like when you spill a can of tomato soup on your trousers, nobody really cares if your trousers are well creased or not.
Then there are photographers, who know what they are doing and their technique in many aspects is near flawless. In that case I would say, it would be ridiculous for them NOT to use an FF format. But now we are talking about the minority.
The majority are cheerfully carrying around these huge FF bodies/lenses not being able to translate its advantages into their photography. That is the "truth".
PS Yes, I can see four cans of tomato soup flying in my direction...