If you are comfortable building web apps like the early adopters did in 1999 that later got mainstreamed with Ruby-on-Rails and related frameworks, HTMX adds a wonderful bit of extra interactivity with great ease.

Want to make a dropdown that updates a enumerated field on a record? Easy.

Want to make a modal dialog when users create a new content item? Easy.

Want a search box with autocomplete? Easy.

As I see it the basic problem of RIA front ends is that a piece of data changed and you have to update the front end accordingly. The complexity of this problem ranges from:

(1) One piece of information is updated on the page (Easy)

(2) Multiple pieces of information are updated but it's a static situation where the back end knows what has to be updated (Easy, HTMX can update more than one element at a time)

(3) Multiple pieces of information but it's dynamic (think of a productivity or decision support application which has lots of panes which may or may not be visible, property sheets, etc -- hard)

You do need some adaptations on the back end to really enjoy HTMX, particularly you have to have some answer to the problem that a partial might be drawn as part of a full page or drawn individually [1] and while you're there you might as well have something that makes it easy to update N partials together.

[1] ... I guess you could have HTMX suck them all down when the page loads but I'd be worried about speed and people seeing incomplete states