I have had a GPS Only Series 4 (2 years), and since my iPhone is rarely far away from me, I have All the features and no extra charges on my cellular bill (in the U.S. that would be $10 each month or over 2 years $240.00 plus applicable taxes and fees).
I have not felt the need to get a cellular model.
My Wife, that does not want to put an iPhone in her pocket all the time, asked for and got a Series 6 Cellular for Christmas (just activated the cellular today). We went for a walk without her iPhone and she was testing iMessage, Walkie-Talkie, cellular calling, etc... While I was walking with her, exchanging the iMessages, Walkie-Talkie, cellular calls on my Apple Watch, because I had my iPhone in my pocket.
NOTE: She had the same model GPS only Series 4 during the past 2 years, and while she often kept her iPhone in the kitchen, she was able to answer phone calls, messages with me and other family members, get the current weather, etc... while she was in the bedroom (maybe 50 feet away on a different floor).
So it was not that should did not get full functionally from her old GPS only Apple Watch, she just does not carry her iPhone everywhere, and did not want to be out of contact if she went for a walk for bike ride forgetting to grab her iPhone.