Very different sort of day to when I last posted, but the Watch battery is still holding up well:
Model: 38mm Sport Silver
Day length: 20 hours
Watch battery remaining: 16%
iPhone 6+ battery remaining: 85%
Watch went on at 1030 yesterday morning, and I just removed it at 0600. Did mild activity today: an hour's walk, and a 30min indoor cycle, but was sitting around at my desk most of the rest of the time. The phone got minimal usage as a result, so the battery hasn't run down nearly as much the day I was out and about.
Considering how well it's held up, though, the Watch doesn't seem to be draining it much at all. I'm guessing that the always-on Bluetooth drain is offset by the reduction in usage for notification for my usage, but that's probably not true of everyone.
Still pretty impressed, overall. The only third-party app I've installed is Twitter, though, so that's probably helping me somewhat.