Haven't seen any major issues in battery on my series 5 44mm cellular so far. I'm jumping from a Series 0 so honestly anything is better battery than I was getting on that, surprisingly Series 0 still did usually get me through a full day though usually, although with very minimal actual use. If I use my series 5 heavily through the day, maps, music controls, weather, calls, texts, siri, etc it'll usually go below 50%. Some of that was kind of "new toy" overuse at times just fiddling with it because of how big of difference it was from my Series 0. I'm about a week into having my Series 5 now though and it and my use seem to be settling in. Through a pretty busy Friday now into Saturday at 3am it's at 54%. I have an easy 10-4 day Saturday so should easily last me through Saturday too.
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Maybe I'm wrong on this but unless an iPhone is out of range on bluetooth and you're off of wifi networks on the watch, cellular should never kick in even if it's still on/activated.