Some interesting perspectives on this thread and I'd bet quality of storage on-site is indeed a big factor in avoiding dirty/wet fuel. But there again, who says supermarkets will be worse than a franchisee in that regard? (And actually in my whole motoring life I've never had a problem with dirty fuel in the UK - only in developing countries where you buy it from the roadside out of rusty old jerricans).
I've always bought basic diesel (from Tesco/whoever) and wouldn't pay for premium diesel just because the fuel company says its better. Well it could be, but how much better?
An extra 13p a litre (
www.petrolprices.com) would mean maybe £200 a year siphoned from my pocket into the fuelco's. That adds 24% to their net revenue, and a huge percentage to their bottom line just for adding an extra squirt of detergent and some marketing guff. So one thing I do know is that they're REALLY motivated to persuade me.
Opinions and personal experience from vehicle owners, engineers, or people in the motor trade, are great to hear (the fab guy who services my cars swears by a proprietary oil flush and he may of course be right). But with any such self-reports the placebo effect (as I think Flying Banana mentioned) is going to be massive. That's why pharma companies have to do double-blind trials before they can sell us new drugs. And why we don't just rely on doctors' opinions any more - well, not since they used to tell tobacco companies that, yes, smoking was good for us.
I just haven't seen any quantified evidence for the benefits of premium diesel. This one looks like a balanced and plausible article, but even then it doesn't put any figures on the benefits:
http://www.oilem.com/premium-diesel-versus-standard-diesel-fuel-2/
My working assumption though is that if fuelcos had any really high quality, quantified evidence about their 'premium' products being worth the extra they charge for them, they'd have yelled it from the rooftops by now. But they don't, so they rely on weaselry like "formulated to protect against wear" or "will potentially make your engine run more smoothly" or "can help your car maintain its full performance potential".
Just my views.