I've used it for about a decade or so. It's really good, but the entire program is tightly coupled with old MS APIs/IE, so importing material is a pain.
Roughly the process goes: Import article -> read section (create extract/readpoint (a bookmark of where you were in the article) -> review extracts/articles -> create cloze deletion/card (if necessary).
I can't use any other app for this process because you can't really schedule material like that in Anki/Polar Bookshelf/RemNote. Supermemo is unique in that the material you want to make cards of eventually gets scheduled itself, so you don't have to remember to read something. I would use Polar Bookshelf, but every time I've checked in on it, they haven't added that feature. In Supermemo can import dozens of articles at a time, assign them priorities, and know I'll eventually be shown them to read.
I have a couple thousand articles imported at this point, and I never need to remember to read them. This is the most important feature for me, and why I've continued use Supermemo for so many years rather than switch to Anki or Polar Bookshelf.
Not needing to remember to read something you found interesting, rather than just save it as a bookmark somewhere (every bookmarking app) is a killer feature for me, but apparently is not very useful to other people. I've tried explaining it to several people (including the developer of Polar Bookshelf), and they just don't really care.
The longer I've used Supermemo, the fewer cards I end up making from material (most things aren't really worth memorizing/remembering or even reading once you step away from it for a few days/weeks). But, I still import a lot of articles/comment threads from HN etc I eventually want to read.
Often times I'll create extracts, and not actually make cards of them, because I just like rereading that particular paragraph.
I have tasklists of business/programming/etc ideas, that also get shown during my daily reviews. I can add to them, or generally get reminded that I was thinking about X at some point. Very underrated feature.
If you don't want to see something again, you can dismiss it from the review process so it doesn't show up in your reviews. However, the information is still in your collection, so you can still search for it.
Most people take notes and then never look at them again (this comes up in every note taking thread on HN). Supermemo continuously shows me my notes over days/months/years. It also shows me interesting articles I want to read on a daily basis. It also helps me remember whatever I want. All I have to do is open the app once a day and hit next repetition.