I have been using an Apple Watch 6th generation for a time, and thought it was time for an update, as I do rely a bit on the health features since having a longer illness, to monitor things. I figured the sensors would have improved by now, among other things. I'll be honest - I see very little difference between my old 6 and the new 9th generation. On the plus side, the speaker and mic seem to be a little better on the 9, as it's easier to hear phone calls, and they say I'm louder on their end. However, on the con side, the battery life on the 9 actually seems WORSE than on my 6, and, the "ping" that taps your wrist when you get a notification is weaker on the 9 than on my 6 (even when set on high).
The new processor on the 9 is marginally better in real-world use. I got the new magnetic larger band with it, so in that regard it's nice they have one offering that fits larger wrists - though I do also use 3rd party stretch bands for comfort. All in all, as a standalone Apple Watch, I say the 9th generation is very nice, but, if you already have a 6 or newer, so long as nothing is wrong with it, I'd say there's not a huge reason to upgrade to this one. There aren't really any new sensors (except one for temperature) and it pretty much does what the last 2-3 generations did.
Side note: I have had the opportunity to use an Apple Watch ULTRA (1st gen), and I can say that if you can handle a larger and heavier watch, that one (and the new gen 2) are definitely an upgrade over the regular Apple Watch. I have large wrists, and the larger screen of the Ultra is a pleasure to use compared to the others - battery life is so much better, as are the antennas, speaker, etc. So if you can swing the $$$ for one and don't mind the size, really, updating from a regular series to the Ultra might be the way to go.