I think one of the biggest problem people have with the HHKB is just the lack of an arrow cluster. I do sometimes wish that the original layout would have arrow keys, but its creator didn't need them for his work, and his original version didn't even have a fn layer for arrow keys.
I bought my HHKB off someone who had replaced it as his daily driver with a WhiteFox partly because he found it cumbersome to not have an arrow cluster. It then took me some time to adjust, which also included me having to install VIM mode on my XCode, which I now use everyday. But I'm not fully adjusted when I use Windows, so I got a Novatouch for home.
I really think it depends on what kind of typing you do and the environment you use, but if it weren't for the arrow cluster, I could see the HHKB becoming more common.
As for regrets, I have absolutely none. I appreciate it everyday and I do enjoy that I have been forced to become a more proficient VIM user, which, is a bigger challenge than getting used to the layout. The quality is great, and the sound and feel of it (it's lubed and 'sphered) brings joy to my fingers and ears. After I got it, I purged all my MX boards because I couldn't quite bear them anymore, so that could be a potential negative I suppose.