I got my brother and grandparents to use them and switch away from supermarket fuel to a big brand (Shell round here, directly opposite Tesco's so usually same price or cheaper).
Both say the car felt smoother and got improved MPG - significantly more so in my brother's case.
As always YMMV.
I do feel a difference in the engine's performance when I use Shell or Jet compared to when I go to Morrisons or Tescos..haven't been looking out for an increase in mileage but guess I should...
Tesco are supplied by Texco and JS by BP and WMM by Esso. All comes from the same refinery in you locale, and all have the same grading structure for purity they are allowed to sell. Do you really think the supermarkets would sell a lower grade fuel? or that Esso etc wouldn't if they could get away with it?
Shell/BP are the only ones who offer anything different in this market. If you really want value for money ask your garage owner about pump calibration.