There was nothing wrong my with Series 5 watch, but for various reasons I needed to upgrade a lot of my business tech, so I got an Ultra in early December as part of those upgrades.
Battery life is really good and the slightly bigger screen is good on the old eyes. The case being larger, I have had to adjust it slightly higher on my wrist for comfort. The old one was just fine for my daily uses, so I haven't really noticed much performance improvement in my use case. Opens/closes garage door, email and text alerts, tracks my daily walks, tracks sleeping. I'll probably try it in the pool next summer swimming laps, I didn't use the old one for that as I didn't have a band conducive to swimming. I may start using apple pay more with the watch. I didn't use it much before as it wasn't accepted at the few places I go, but that seems to be changing.
I did like the new Ultra watch bands being larger, but buying new did cause me to look for new bands, which I should have done sooner. I really needed one more link in the metal band I had for my Series 5. I found these on an Amazon lightning deal over the holidays and really like them.
Amazon product
They work well on both watches. I gave the old watch to my mom for the fall protection, general alerts and being able to call with it. I needed a stretchable easy to put on band for her and these work great for that and are really comfortable.