We both went from a Series 0 (original, pre-ordered), to an S4 for her, and several months later, an S5 for me - the S4/5 are the same other than the OOD of the S5 (it might have more base RAM too).
Neither of us have had any inclination to upgrade, they get pretty regular use for fitness and health, including heart rate/pulse, lots of general use, just haven't seen anything in the next two generations after mine that really motivated me. The O2 function is potentially good, but apparently kind of mediocre in implementation - both of us easily get a long, full day, with constant use - I think there's had to be some kind massive breakthrough in battery life or some seriously improved and/or brand new sensors (glucose) in the near term. Now in the longer term, I imagine like our old watches, old iPhones, they'll start running out of processing power to keep up with the OS advancements, so in the slightly longer term, I'd imagine an upgrade will be on deck.