It's the worst possible financial idea - history shows that financial transaction taxes simply drive business elsewhere. The only people who end up paying are those who are captive to the county in question - those too poor or financially unsophisticated to take their business overseas. AKA the lower and middle class.
So if you want to stick it to middle class investors, a financial transaction tax is great. Otherwise, it's a bad idea.
The failure of the Swedish transaction tax is a perfect case study.