For a more accessible discussion, look at WorldChanging.
The economic arguments against revolve around the use of two constants, delta and eta, “The characters are Sir Nicholas’s shorthand for two concepts. Delta determines the weight he places on the welfare of future generations that are not yet here to stick up for their own interests. Eta governs his answer to a different question: how much weight should be given to the consumption of the rich relative to that of the poor?”.
The critisism focusses on the choice of values for these constants – the assumptions made will no doubt lead to endless debate and mean that the report is essentially one scenario of many (much like the IPCC climate change prediction scenarios). Depending on how you select values, the burden of costs vary and so do the benefits. Making policy based on the report should certainly take into account the assumptions leading to a particular scenario.
The more pro-angle takes the view that however you might split hairs about the actual costs, these will most likely be small in comparison to future savings – especially when you liken the spending in terms of insurance against potential catastrophic costs.