I have just used a tank of diesel with Millers added (2008 GT with big miles) - 581 miles from the tank gave me 47.5mpg against my previous best of 541/46 and my average of 44 (80 miles a day commute, mix of city, A road, DC and motorway)
Here's the maths:-
Seeing as the Millers costs about £13 and does 10 tanks, its simple arithmetic to work out it costs about £1.30 to add to a tank.
I got 40 miles more from the last tank of fuel compared with my previous best (about 70 miles compared with the average). Each mile costs £0.13, so I've benefited by £5.37 across the tank, even when comparing against my previous best.
Deduct the cost of the additive and I'm just over £4 in profit
So - on the basis of the first tank, I'd have to say the additive is worth it. Engine seemed to be a bit smoother and quieter as well, but it could be a placebo effect.
Next test is to try a tank of V-Power, which will be about £4.40 more expensive for a tank compared with standard diesel - can't see it giving me any more benefit than the Millers
Final test will be to have it remapped for economy (don't need any more performance)