I went to the Galaxy S9+ back in June when there was a Best Buy sale offering a few hundred dollars off it. So far, I'm really enjoying it.
Not crazy about the glass exterior (even though I have a case anyway), but it runs smoothly and faster than any phone I've had in the past. The Samsung Experience skin is also so much better in comparison to the cartoony Touchwiz days, which was a huge reason why I never considered them in the past.
I loved the HTC 10, but there are a few little things that make the S9+ seem a bit more of a smoother experience. For example, when I plugged the 10 into my laptop via a USB cord, moving the phone ever so slightly could cause it to disconnect and make me have to unplug and plug the phone in all over. That hasn't happened with the S9+ at all. There's also no slight hissing when I'm playing music via headphones, which I always heard during the more quiet or silent moments between songs with the 10. Maybe something DAC-related there?
There are some software tweaks that I find helpful as well, but might just be attributed to the newer technology that phones have every cycle or two (like telling you when apps are taking up power automatically, etc.).
Truth be told, I don't miss a whole lot about the 10, although Sense was my favorite skin for Android. But after hearing they didn't update it much for the U12, maybe I didn't miss out on much overall? Plus, it's nice having a few things that the later HTC phones were doing without like that headphone jack. In any case, I'm just lucky that moving on from a great phone like the 10 went as seemlessly as it could. I still have the 10 as my back-up device too in case the S9+ breaks at some point, but hopefully I'll still have it running okay in the future!