I used to use Shell Optimax, then for a period of time BT Ultimate, and now I am using the less-advertised Texaco High-Octane. I haven't used 95 RON in two years now.
The main reason for me is that the fuel is supposed to be cleaner and the added detergents. However, I still add Redex with each tankful for good measure.
I don't know if there is any significant improvement in MPG as I don't actually check this very often. On the performance side, I seem to think that the engine feels as if it has a bit 'more bite' with the Texaco compared to BT and Shell, but nothing I can measure. At any rate as I said out and out performance is not my main concern, the longativity of the engine is (I had the car for 6 years and may keep it for a few more...).
And if you were wondering why I swapped between petrol companies, well it is simply down to logistics, and at the moment Texaco petrol stations are easier to find on my usual routes.