How belligerent are the independent expenditure groups? Very: Republicans spent more fighting Romney than Democrats have backing Obama.
Voters dislike negative advertising. And voters don't like super PACs very much. But perhaps I repeat myself. Check out this animation of spending by super PACs over time from Northeastern University's Lazer Lab, last seen in this space with another infographic on fundraising. This video breaks super-PAC money down by both partisanship (Democratic vs. Republican) and tone (negative or positive). The video has sound, but I actually recommend listening to it without, unless you enjoy cacophonous cheering and jeering. The figures run through September 13, the latest figures available from the FEC.
There are two things worth zooming in on. First, look at how negative the spending is. Republican super PACs have spent three times as much opposing Obama as they have backing Romney, $46 million to $14 million. The gap is even larger on the Democratic side (though the absolute numbers are much smaller), where there's been nearly $28 million in attacks on Romney and only a little more than $3 million in favor of Obama. The most telling stat -- both in terms of how negative super PACs are and how much Democrats are being outspent -- is that Republican super PACs spent more trying to sink Mitt Romney during the Republican primaries than the president's Democratic allies have spent in favor of him during the entire campaign, $4.7 million to $3.2 million. Here's a screenshot of the state of the race on September 13 (click for larger size):