I had used Feedly for several years, but made the switch to Inoreader last year, in part because it has one additional useful feature that Feedly doesn’t: OPML subscription. While it’s nice to be able to import and export OPML files, needing to remember to update them can be an unnecessary step, particularly if 20+ people need to do the update to capture all the new RSS feeds added. (As an example, say one or two students join a class late and everyone has already got the original OPML export and now needs to update to add a few more feeds to keep track of classroom activity.) OPML subscription improves this by allowing the subscription to an OPML link with multiple feeds in it. If the original OPML file updates with new feeds, then the reader automatically updates them and pushes them out to everyone subscribed to that OPML file!
Think of an OPML subscription as an updating subscription to a bundle of RSS feeds which all also provide their own individual updates. Instead of subscribing to a bunch of individual feeds, you can subscribe to whole bundles of feeds.
For those looking for some sample OPML links to subscribe to, try some of mine which are listed at the bottom of the linked page. For some ideas about building your own data stores with OPML links for WordPress, try my Following Page solution. WordPress’s old Link Manager described on that page will provide the ability to store the data and provide the OPML links, the rest of the page discusses publishing it on one’s site so that it’s publicly available if you wish. URL schemes for sub-categories are discussed separately.