I personally think that, from a moral standpoint, it is essential that the perpetrators of the financial crises repay governments and thereby citizens for the massive costs they have effectively forced upon us. Therefore, as well as a tobin tax, there should be other measures like separating deposit banking from the gambling which predominately caused the problems in the first place.
But as people above have highlighted, there are real practical concerns about such policies. For example, what the extra money will be used for, what will happen if it is applied only in isolated areas, etc.
As a consequence, I think it needs to be levied globally. Secondly, whilst particular governments may be wasteful spenders, they do spend (supposedly) for the benefit of citizens. The super rich and the banks do not, and more money for them will only go in bonuses to the very people who have caused these massive problems. Or it will go to shareholders who are supporting and endorsing the ponzi-schemes that these people operate.