I've been mostly using emacs past 30 years ie. about the time when system memory wasn't any more constraint which while single user was about 8MB at least. But I did earn my living before that about 7 years mostly using vi as most usable editor in the system and that 8MB was luxury most of that time.
But even emacs IMHO was and is vastly superior, vi still had it niche fast small edits and especially before log based transactional filesystems. After power outage or bad brownout event system crash there was great chance you got to fixing filesystem with fsck (which did often take lot of time) and worst cases finally debugfs trying to fiddle bits that you get fsck fixing rest.
Bringing system up with old system could be tedious. Before you get system enough up single user mode and just root fs mounted you had to resort you way forward using those modest tools you had there. It was really great if vi did work, but it too required sometimes more memory than you had before swap was active. If not, then ed was your friend, ex is just vi without visual mode.
For a long time vi was also able to edit very large files. It did not require reading whole file in memory before it allowed editing as for example emacs did (or mmap's it memory later).
These days I use vi for quick edits like someone above mentioned and like it more than any later replacement (nano etc) if emacs is not there, not worth installing it for just quick change or when can't install on (embedded) or someone else's system for any reason.
Vi is often available also *bsd based appliances which I've been using like Junos, Netscalers, etc.
This is about Vi though and that's certainly more rare. For me that's only used over a rare ssh with vim not available but vi present