It probably depends on the car but I agree with those that say better fuel
is better.
Yes, all modern engines will adjust themselves to lower octane and run fine, but they will do this by "detuning" themselves - retarding the ignition etc. With a higher octane fuel they can run more efficiently (ie more power and/or better economy). That is basic physics imho.
Whether you will notice it in driveability and economy depends on you as much as the car - the affect can be quite subtle. What I can say is that with my 6 (which has done 120k miles) the affect is noticable. I've tried both shell V power and their normal and the v power gives noticably more low end grunt and better fuel economy round town. (On motorway journeys I can't tell the difference) I've yet to do proper sums to work out whether it is cost effective though.
As to additives, I am not so sure about the different fuels but my 6 was starting to hestitate as it went through 3k rpm. This got worse to the point I decided to do something about it. As it happens somewhere local was selling redex petrol treatment cheap so I decided to give it a go. Now I am in general very cynical about such things and had no expectations that it would make any difference at all. But it did, after a couple of tanks of treated petrol the hestitation the hestitation is gone. Whether it will stay gone when I stop using the additive I don't know.
Last bit of "evidence". A mate used to have a MG ZS 2.5L. He ran it from 30k miles to about 100k miles and, over a period of 4 years logged all his mileage and petrol use/cost. 2 years on 95 ron, 2 years on super. His spreadsheet shows that the super unleaded saved him 10% on the petrol cost. ie. increased in cost of the super was significantly outweighed by improved economy.
Now these are the results from just two cars so don't prove any affect on other cars. But it does prove that anyone who states that fuel brand/octane rating will not make a difference on any car is wrong