I'm happy with it. Last night after having the Watch on from around 8:20am until around 11:30pm, I still had over 60% battery left. I didn't work out yesterday but on days where I do, the lowest I've been down to is around 47%.
On an overnight trip, I once brought my Watch without the charger and put it into power saving mode when I went to sleep that night. By the time I returned home and put it on the charger, I still had somewhere around 13-15% battery left, so it was basically able to last me two full (workout-less) days.
For transparency, mine is the 42mm version. I had the 38mm for a couple of weeks and generally I would probably end the day with around 20% less battery than I get now with my 42mm; like under the same conditions for yesterday, I would probably have like 43% left on my 38mm vs like 63% on my 42mm.