Still trying to decide where I stand on this kind of thing, I don't have an ideology which tells me what to think in this situation.

The tax should definitely be at least enough to cover the externalities, and in this case I expect it won't exceed that amount (of course it's fuzzy, because you have to decide how much of the obesity 'epidemic' is caused by sugary drinks and how much it costs).

But if you have the choice of either getting $x from income tax, or $x from a fat tax, then from a purely economic perspective the latter is better, because you also save $y from the reduced level of obesity that results (less health expenses etc.). I guess it really depends how you feel about social engineering... I'm not against it in principle, as long as the benefits are clear and the manner in which it's done is not too oppressive. The smoking tax is a good example of social engineering being put to good use.

So yeah I'm for this, but could be persuaded otherwise.

One thing to add to that: ideally these kind of tax increases would be coupled with a lowering of the sales tax, to reduce the regressiveness of the tax. If I understand what's going on correctly, this is a great time to be lowering sales taxes, as long as the revenue loss is balanced out.