The trouble is, each installment is less effective then the first.
It dilutes the currency strength by increasing inflationary pressure (more money floating around the economic cycle technically makes it less valuable).
The idea is that the extra money is used by banks and other various organisations/firms for investment purposes in the economy. But the government has no control over the money apart from the initial purchasing of the assets that the QE buys. So once the money is in the circular flow, its providing extra liquidity but many organisations could take that money and invest it abroad in growth prospects like the BRIC nations.
We will need to properly evaluate it over time when there is plenty of data available.
In the meantime, people that support it will say that it has helped keep our heads above water and prevented us from slipping into a steeper recession; people that oppose it will say that the economy has flatlined anyway so it hasn't done any good.
It's like saying how effective is the Coalition's economic policy, if you support it you can argue it has stopped things from going worse, if you oppose it you can say we are back in double dip.
One thing which hasn't transpired though is the "sky rocketing inflation" that was predicted by some of the opponents of QE when it was reintroduced last autumn, inflation at the time was about 5% and some people were warning that it was "Robert Mugabe economics" and we would see inflation spiralling out of control, but inflation has fallen since then.