I just replaced all four of the tires and the shop accidently put regular tires on and I drove on them about a month before they called me up and told me and I went back in to have the run flats put on. I did notice a slight difference in ride but did not measure the mileage.
We had a flat one time and we were able to drive to the dealer to have it replaced. It was very convenient and I would give up the slight difference in ride and mileage for that.
That said, most cars that have these do not have spares, so you would be taking a chance not replacing them with run flats.
I tend to favor a firm ride suspension and Bridgestone is the only tiremaker with a run-flat tire that is intended for cars that were not equipped with OEM run-flats. They are reported to be a little less harsh than the tires made for OEM replacements.
I’ll post my impressions when we try them a few miles.
They ride rougher and are noisier. I would never go out of my way to buy them. There are a lot of quality tires out there. I actually have Nokian One tires on my sons car. They have extra sidewall strength to help with pot hole blow outs if that is of interest.