I don't but I do use, and am interested in, their software. So it's a bit disappointing to see a link to their site while using hn on mobile, only to get to a page that doesn't display anything useful.
You seem to assume that everybody on this website knows what Ring UI is. But I didn't know and judging from quite a few other comments here, I wasn't the only one. All we had to go by was the title "JetBrains Ring UI" and a comment stating that a date picker looks nice. Without any kind of description it's not that weird to assume a website with a datepicker works decently on mobile.
I'd totally use them on my mobile devices if versions for them existed. They're just too convenient even though they lack a keyboard. Wish I could use them to write software.