It seems to depend on your plan, and not all plans will allow the Watch to be added. Assuming you do have one of the compatible plans, it'll be at least an additional $10 per month. The advantage is really just that you can still be connected to the internet without having to carry around your phone. So you can send (if you're OK with dictation) and receive text messages and emails. I believe you can also do phone calls but I'm not 100% certain on that.
If you're a surfer who wants to be connected while over the ocean and carrying around more gear is a liability, then it'd definitely be worth it. If you're looking for a reason to leave your phone at home more often then it may be worth it. If you tend to forget your phone often and need to be connected, it may also be worth it... but it'd probably be cheaper to figure out a better way to not forget your phone.
Of note, you can use Apple Pay from your Watch without being connected to the internet and without an iPhone nearby. So if that was anyone's motivation for getting the cellular version with a cellular plan for it, save your money.
My take is that it's a neat option, but $10 per month - or $120 per year, which is close to a third the cost of a new Watch (or almost the cost of a new Watch that is from the previous generation, as sold by Apple) - is a bit much to ask. Then you have to figure that you're already paying at least $100 extra to even get the cellular model over the non-cellular model. If you've spent a bit to have a nice iPhone then it's almost insulting to realize how much more you're paying just to have the cellular-connected Watch, where the cellular functionality really only comes into play when you don't have your phone with you. That's a lot of money to be spending just to leave a nice phone unused and wasting away at home.
Personally, I use the Watch more per day than I do my iPhone. I read and dictate text messages, check emails, access Reminders lists, and a few various other things from my Watch. Yet I couldn't bring myself to get the cellular version and spend $10 per month for connectivity that I'd barely be using. $10 per month doesn't sound like much, but it's a total waste when I consider that I'm already used to bringing my phone everywhere, and that I use my phone for more than the functions that my Watch has usurped. Things like CarPlay and camera functionality are things that a cellular-connected Watch can't do. I don't begrudge anyone who feels liberated by their cellular Watch, but in my case - as well as that of my family and most of my friends - it's a total waste of money. If the cellular companies wanted to charge $1 or even $2 per month for connecting the Watch to their service I might buy in, but not "at least $10."