I wonder how long this is going to last for, to me it doesn't make sense from an emissions point of view. For people who would have bought a car with low emissions to save money on tax might as well just buy a higher powered car that pollutes more but cost the same in tax a year.
It is more likely to make me go and buy a car that was £300 a year to tax, instead of a car that was £30, as I am "Saving" £160 instead of "Spending" £110 a year.
In a way it could be a good thing for me, as I am more likely to own a sporty petrol instead of my chuggy diesel.