Just walked from Lisbon this June. What can I say about the Lisbon to Porto stretch? ...
Well, I think it was worth doing, but unlike other Caminos, I don't see myself ever wanting to redo it. Out of about 14 stages, I would say much less than half of them had picturesque scenery or a historically interesting town at the end of them, so if you're doing it expecting to enjoy the nature or towns, I think you might be a little disappointed.
That's not not so say that places like Tomar or Santarem aren't lovely in their own right, but all the nice places on this stretch are spread out days apart, so there's no real way of saying "I'll start further on and miss out the ugly bits". The ugly bits are evenly distributed across the whole two weeks.
There's also a lot of road walking, and this really got to me. After Coimbra there were several days where my feet felt only cobbles and tarmac and this was causing me feet problems that I've never had before on any of my four other caminos.
It was also a bit lonely, yeah. There were always people in the albergue with me, but often the social aspect just didn't "click" for me. Quite a few times on this Camino I wondered just why I was doing it as I thought I wasn't getting much out of it - socially, spiritually, physically. But maybe I was just unlucky. I did meet others who loved this stretch, though equally some had enough and quit. They say the camino gives you what you need, and maybe I just needed to learn to be more independent and resilient.
Oh and weather-wise... I had 37-38 degrees for several days at the beginning of June, so don't forget that you are significantly further south than all other Caminos other than the VdlP. You will feel it in the sun. It was punishing.
I never felt less than welcome as a Pilgrim in Portugal and found the people even friendlier than the Spanish.