Going to work, listening to music, replying to messages, going for a run and then stopping by the shops to pay for things...all using my wrist. It's a great experience. But I have to keep my phone nearby. And LTE watch sounds great.
There are a lot of UX issues with the watch that makes it frustrating at times. But it's getting better and more capable every OS update.
Honestly, I really don't see the benefit of going through the additional burden of getting a new cellphone plan, SIM, paying more for ... what? One advantage is definitely that in emergency situations (i.e. on a run without the phone, hike etc) you can now call from the watch. But I just don't see any other benefit.
I don't think that you will need a new plan, I think that it will use your current plan.
Or did I get the wrong end of the stick?
They also can't hold a charge very long. If I charge mine up overnight, by next evening when I go to use them, they're already half dead. Great product except for very poor battery life.
I may be in a minority, but over the past few years, I've been trying to actively reduce the amount of times I pull out my phone. I feel like it will be liberating to leave home without your phone to go to dinner or go for a run, but still connected through the Apple Watch. Although it will still display notifications, I personally won't be tempted to browse Twitter or Instagram when I have a free moment.