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This is obviously a contentious topic, but I'd add one thought into the mix: don't be swayed by advertising hype. Instead, take an easy step and try if for yourself. Then use your head, your hands, feet and bum-on-seat to make up your own mind. If you don't experience any improvement, go back to bog standard.
In my experience over the years, my E46 M3 responded well to Shell Optimax (highest octane I could buy locally back then). The difference: smoother and and quicker throttle response and a useful bit of extra go at the upper end of the rev range (circa 6,500 to 8,200). No significant gain in power or torque lower down. But, for the smoothness and responsiveness alone, I went Optimax all the way. The difference wasn't a 'slap in the face' thing, but very obvious to any keen driver tuned into their car. Given the way I drove that car, my interest in economy was zero - never checked versus standard fuel.
More recently, E39 530d M Sport (tuned with hybrid blower, re-worked intake system, Eibach anti-roll bars, springs & AP Racing brakes - 300BHP and 470 ft/lbs). Tried both Shell V-Power diesel and BP Ultimate diesel. Both made a noticeable difference, the most obvious change being smoothness and consistency of pickup. There was more grunt in the 2,000 to 3.500 rev range and I consistently got 3-4 more mpg, especially on a long motorway run. Again, I wasn't especially into making a saving and I'd guess I was level-pegging on cost at best, or more likely a bit down, but the drive was improved to the extent that I would always look for one of these fuels first and take normal fuel only as a fall back.
As someone else mentioned - it's not just he extra cetanes, but the cleaning agents are worth taking into account if you care about the car. I learned the value of this recently when I had all my injectors replaced on the E39 - made an incredible difference.
One other thing - if you make the switch, you won't feel all the benefit until you've run through 2 or 3 tanks.
I guess some of you will be bike riders and here you can multiply the impact of premium fuel by a factor of two, just because bike engines are so much more highly tuned (180 BHP from 1300 cc 4-pot engine). I have a BMW K13s and it loves the premium fuels. I never find much difference in power, but there is a big difference in smoothness and consistency of throttle response and, if you ride a bike, you'll know how much that matters. One time, on a touring trip in France, I was forced to fill up with 95 octane fuel (on an older K1200 R Sport) - I got half a mile down the road, lost power and stalled. All my mates had to wait while I tried to restart the bike - got it going, but it ran like a dog for the next 100 miles. Never, ever, ever again in a fast bike ...
I'd guess none of this stuff makes a saving overall , unless you take the long view on keeping the system clean and reduced long-term maintenance costs. For me, it's just an easy way to get a better, more enjoyable drive and, with the gain in economy, you're not out of pocket by as much as the difference in pricing would suggest ... worth trying for a few tanks.
Dave
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