Well, it isn't a phone replacement and not designed to function in the way you described. I didn't know what to expect when I bought mine, honestly. I bought it during the holiday period so that I could return it after the new year if I decided I didn't like it. It only took a few days for me to appreciate the watch. I will be keeping it. But I never expected to use it on LTE 24 hours a day, as you stated. There are a few things it's good at by itself, but it mostly compliments the iPhone. I have run to the store (and went on runs) without my iPhone and it's worked fine then. But I wouldn't expect to leave my phone at home and use the watch as a replacement.
I don't consider myself a heavy user, so my usage may not be typical, although it seems most people on here are somewhere close to describing the same battery life as me. I just used mine two days in a row and charged it last night. After 48-50 hours of use, it was at 23%. Some of that was on its own (away from the iPhone). I put it in theater mode at night, and never disabled LTE, GPS, Wifi, or Bluetooth (except for a 4-hour flight when it had to be on airplane mode).
If battery life is your biggest hangup on getting the watch, and you don't consider yourself a heavy user, I would say go for it. There is a 14-day return policy, so you'll easily get an idea if the battery life will be an issue for you.
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It appears the app is tracking my heart beat all through the night, it appears to be checking in 10-15 minute intervals. (Apple Watch series 3 though). Not sure if series 1 has an issue.
