Tagging @Jim McClain as well, I'm pretty sure I recall him using it during an earlier conversation about DAMs.
The last time I personally used it was when back in the day when it started life as a little shareware image viewer that was perfect for using it with usenet (let my packet reader slurp up a usenet feed overnight using my dial-up connection, check out the next day with ACDSee what images were retrieved). I totally missed the transformation of it switching from a file viewer to an image editor and have not tried it in years.
Yes, I use ACDSee (version 19 currently), but mostly only as an image file management tool. Very rarely have I ever used it to edit an image. I am a fan of Adobe's Photography Plan (Lightroom & Photoshop) for that. I have tried using Lightroom and Adobe Bridge for asset management, but neither have the ease or versatility that ACDSee has, in my opinion.
 And I bought it. Things I like about it...
1) Its not a proprietary database, so changes you make are to your originals, better make copies to work on.
2) You can send to your other editing software with a right click on the photo in view mode.
3) Easy ratings system. Now all I need to do is actually use it.
Things I don't like: Just one so far. It is struggling with accessing my photos on a drive located elsewhere on the network. This may be a problem with my wifi, or the fact that there are currently FIFTEEN other wifi users in the near vicinity, all with their own networks, and things seem a bit congested. I should probably break out the Airport Extreme Base Station which is old, but reliable, and see if thats an issue. It also isn't that happy on my 2010 MacBook (White). Lots of beachballs. So the next step is to install on the mac mini (2012 model, running Mojave), when I can bring myself to go into the study, which is COLD.
Further to the install on my Macbook which ACDSee didnt seem to like much, its now running happily on my 2012 Mac Mini with a standard HD (the Macbook has an SSD) and 16GB RAM. It was fast to load, and fast to find thumbnails in an external drive (the standard install disallows removable anything so you have to change that in prefs). External editors are easily added, and you can change the order so as to have your favourites at the top of what (in my case) was quite a long list, once the Nik software and Creative suite software was added. I have RAWPower set as the default editor, but also have GIMP and Acorn in the list of usable apps.
It feels "light" by comparison with OnOne RAW 2018.5 which I installed and uninstalled after playing with it for a very short time, and also by comparison with Corel Aftershot Pro 3. Its faster.
For those not willing to spend more money, theres always XNView which is cross platform and which is also pretty darned fast and allows edits in defined external editors.
I'll be using ACDSee on the Mac Mini, but the Macbook... meh... think I might just sell it and be done with it.