I agree that auto sleep tracking is extremely handy, however, it's hardly anything new. Virtually all activity trackers on the market do this; in fact I'm pretty surprised the flex didn't. I'm guessing it was the last fitbit holdout, because it was their cheapie budget option.
Honestly, I think one of the bigger issues with auto sleep tracking on Apple Watch is that you still have to charge it daily anyway (which I usually do right before bed). If the apple watch could even get 3-4 days on avg then auto sleep tracking would be much more useful.
In fact (this is way off topic), but I'm realizing that vast majority of things that I use my apple watch for (activity tracking, run & workout tracking with HR, sleep tracking, basic notifications) are accomplished just as well, if not better, by other, cheaper options on the market that have battery life in the range of 4-7 days. I like my Apple Watch, but being realistic it's really not the best wearable for me. Part of the reason I got it was for future growth through OS updates and new apps/API's. I'm still hopeful that this will pan out, but I'm not as certain. I just bought the Garmin Forerunner 225 for my gf as a Christmas present for $220, no tax/shipping (half the price of my Apple Watch). It has activity tracking, automatic sleep tracking, built in GPS, enhanced run metrics/tracking, HR monitor, stand/movement reminders, and a battery life of ~7 days. Being honest with myself, that's probably more the device for me.... Things such as touch screen, force touch, 3rd party apps, glances, etc is all fancy, but most of that stuff I don't typically use.