It's a pretty good API. I recently built an appointment booking form and used Temporal for it. Thankfully, all the date logic is server-side (in server-side actions on an Astro site) so I didn't have to worry about browser compatibility.

I just have a few small nitpicks, but they're pretty easily fixable by using my own classes that extend the standard Temporal ones, or just standalone functions. Some examples:

• I wish the classes like Date, DateTime, ZonedDateTime, etc had methods like isBefore and isAfter to compare to another instance. For some reason, I can never remember whether compare returning a negative number means the first argument is less than or greater than the second one. Maybe one day we'll get operator overloading.
• I wish there was a way to set the default calendar. I had to manually pass calendar: 'gregory' in a bunch of places because I ended up hitting issues without it (one example). My code isn't running outside of areas that use the Gregorian calendar, so setting it as a default in my code would be ideal.

Node support was a bit rough in June/July so I ended up using the polyfill, but I think they've fixed all the major issues now.

Not that it fixes everything but we'd have fewer ulcers in the world if we just stored every date in UTC and only converted when a human looks at it.