My watch upgrades have been mostly driven by necessity.
Series 0 - was given to me by a coworker two years after launch. It was god awful slow and the battery sucked, but it was enough of a proof of concept that I changed my mind from meh to yeh.
Series 3 GPS - Huge upgrade from the 0 obviously, worked great for a few years. I started running and wanted to leave my phone at home, which kinda worked but loading podcasts and music onto the watch was a coin flip, and it wasn't really safe to be out miles from home without a phone. Eventually even updating the OS became a complete pain in the butt because of the tiny storage, requiring unpairing and repairing... turning a 30 minute process into a 90 min one.
Series 6 Cellular - Massive upgrade. All apps work well and super snappy. I will never go without cellular again; it's so nice to be able to leave the house without a phone and still have streaming music, podcasts, phone, text, and Apple Pay. Always-on-display is surprisingly necessary for me now. I could see myself using this one as long as it's still getting updates. A new model would have to have a really special new feature to get me to upgrade before that.