With about a week to spare before Drupal 7 reached EOL, I managed to rebuild a very large (6,000 pages-plus) and complicated D7 site in Backdrop.
Here it is: https://steampunk-explorer.com/
My feedback is 99% positive. So first, thanks to the developers of the core CMS and the folks who ported the modules. Also to a few people who posted helpful answers in this forum when I ran into problems.
I don't know what I would have done without this CMS. Neither WordPress nor the later versions of Drupal were good options. I am not a professional developer. My background is on the editorial side, and my approach to building websites is to learn just what I need to get a project done. Because Backdrop is so similar to D7, the learning curve was minimal.
This was not a port, but a rebuild, but I was still able to use a lot of the old D7 site as a reference for building the new one.
Most of the glitches I encountered related to gaps in my own knowledge. I was up to speed on CSS as it existed about seven years ago, but I'm still getting my head around flexbox. There are some spacing and navigation issues I'd like to fix, but my first priority was getting the site up and running.
The one feature in Backdrop that caused me the most problems was CKEditor 5. It just seems extremely temperamental, though I've managed to work around the issues I encountered. I did learn that what appeared to be a bug was actually a feature: If you have multiple unordered lists with different classes, it collapses them all into a single class. My workaround is to set all fields that use unordered lists to Raw HTML.
One D7 module that didn't make it into Backdrop was SimpleAds, which I used for managing ads on the site. All of the advertising is locally hosted - I don't use any third-party adtech. Following a useful suggestion in this forum, I used Views to set up a series of advertising blocks. Each has multiple ads that rotate based on a random sort. However, I have no way to track performance of the ads - I can't tell how many people are viewing or (more importantly) clicking on them. If someone is aware of a module that would allow me to easy track clicks or taps on a specific piece of content, please let me know.
But all things considered, the CMS was a godsend, so thanks again to everyone who made this thing work.
Really good feedback. Thank you. If you don't get along with CKEditor 5 you may want to try TinyMCE. I'm shifting most of the sites I manage onto it.
For SimpleAds you could add a request to https://github.com/backdrop-ops/contrib/issues or if you do not have a GitHub account and have no desire to get one (though one is useful if you want to raise issues for Backdrop or contributed modules) then someone can raise the request on your behalf. There is no guarantee but sometimes people port modules if there is enough interest.