I get mine in bulk (a lot cheaper) from Opie.
http://www.opieoils.co.uk/pv-68896-millers-oils-diesel-power-ecomax-fuel-treatment-5-litres.aspx
Fortunately a site sponsor too.
Been running on it for years (used to be called diesel power sport 4) a number of years ago.
Uprated cetane rating, runs smoother plus loads of other goodies no doubt thrown in there too.
I found the stuff in short gives a bit more power (marginal), plus 2 or 3mpg (straight off when I first started using it) and slightly quieter/smoother running. Purely a personal opinion but in twelve or so years of the stuff running through the Leon, it never has had any EGR issues. Obviously not controlled laboratory conditions!
On a simple maths calculation based on an additional 2.5mpg, that 5 litre container gains another 2750 miles. On a very regular average of 45mpg (not great because work isn't far and its very stop start traffic - EGR???) I reckon it saves over £300 based on £1.15 per litre so in my opinion very cost effective in 5 litre form. At about £70 with forum discount code, probably £250 net saving on fuel efficiency alone.
Realistically though I just look at the stuff as raising standard fuel to BP Ultimate/V power etc. Running on that stuff too has positive effects on mpg and running in general from those I know that use that stuff instead of just adding an additive.
No doubt each one could argue that their's is the best product on the market be it straight from the pump or via an additive.