Citizens United is only part of the story. SuperPACs are a result of two cases. Citizens United and Speachnow.org vs the FEC.
In a nutshell, Prior to Speachnow.org, individuals could spend unlimitied campaign funds as independant expenditures, but two or more people banding together could were limited to $5000 each. That means a "millionaire" could spend as much money as they wanted to influence a political campaign, while poorer indivuals were limited by how much they were limited to spend as a group.
This, really, is what allowed super pacs to be created.
BCRA tried to stop corporations and unions from spending unlimited money on "electioneering communications". This was found to be unconstitutional in Citizens United vs the FEC. The court did find, however, that BCRA provisions requiring disclosure of funders were valid.
The ruling even pointed out:
If the First Amendment has any force, it prohibits Congress from fining or jailing citizens, or associations of citizens, for simply engaging in political speech." He also noted that since there was no way to distinguish between media and other corporations, these restrictions would allow Congress to suppress political speech in newspapers, books, television and blogs"
An interesting comparison is Hillary: The Movie (The topic of Citizens United) and
Fahrenheit 9/11. Both were produced by corporations. One a for profit media company, one by a non-profit PAC. Both were created to influence an elections. Fahrenheit 9/11 to defeat bush, Hillary to defeat Clinton in a primary.
One was against the law, one wasn't. Moore's movie was technically legal because it was deemed a "commercial activity."
In the end, I think concern over super PACs is indeed overstated. For every "corporation" wanting to spend money on "electioneering communications" there is a Union or other entity willing to spend money to deliver the oposing viewpoint.