For what it is worth, I ran a long-term activity test on my watch overnight. I have the 42 mm version and set "Activity" to "Other" and the next morning the battery had dropped from 100 to 10% in a nine hour period. This matches a shorter test I had done the previous morning for two hours (10% drop/hour).
This is better than I would have expected and, based on my daily routine, I could actually use it that way because I have ample time to charge each morning and a couple other times during the day when I'm not doing anything. But not for most people.
Also, I don't know how the different Activity options work. For example, if I do an outdoor cycle, there will be the GPS connection with the iPhone. But would an indoor cycle simply not turn that on and save battery on the watch - or just the iPhone? And I have no idea whether "Other" involves GPS or not.
I also have to see how much this drains the iPhone battery. It did a lot last night, but I think the screen was on (fairly dim) a long time so not a good test.