I think this is a terrible decision for several reasons.
It re-enshrines the ridiculous notion that corporations are the equal of 'people.' I sincerely doubt when the founding fathers wrote "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances," they intended this to apply to legal fictions, but rather to real natural persons. Can a corporation exercise its freedom of religion? Can it go to church on Sunday? Can a corporation freely 'assemble' to demonstrate against the government?
Furthermore, before someone claims "but corporations are just associations, and we have the freedom of association," it might behoove them to realize that modern corporations are, by and large , involuntary associations. In any other kind of association for political purposes, all contributing members are equal, and fundamentally agree with the positions being advocated (even if the individual members may disagree about one or two minor issues.) If an individual decides they no longer wish to support the causes the association advocates for, they suffer no intrinsic harm by leaving the association (which can hardly be said for losing your job.)
This was the entire reason for "PAC"s in the first place. The vast majority of real persons who comprise a corporation, including its workers and investors, will have no say in or over how such funds are directed. These decisions will be made by a small handful of people whose political speech might not reflect the beliefs of the vast majority of shareholders or workers. (And might not even actually be in the real interests of the company as a whole!)
And while one can glibly say "well a shareholder can simply divest if they don't care for the speech," that ignores the fact that most investors don't invest directly in corporations, but rather in funds (pension, retirement, etc.) which further distances and insulates the corporation from being responsible to the funders of the speech. I don't know about you, but I suspect that most workers who rely on their pensions aren't interested in ceeding other essential rights to some hedge fund manager. Furthermore, by the time you find out that some corporation has supported some candidate or issue you don't agree with, it's usually too late and your rights have already been trampled upon.
The same involuntary aspect can be said to apply almost equally to unions. Which is one of the primary objections conservatives have to unions in the first place!
Corporations are not human beings, and it is only by the equivocation of them with natural persons that allows this decision to make any sense. Corporations have no natural life-span, have no intrinsic morals or empathy, have no motives other than the profit motive. Furthermore, what interests a corporation has may be in direct opposition to real persons. It may be in a corporation's interest to fund a candidate who supports their position on squelching pollution regulation, for example. While this is fine for the corporation, it does little to soothe those living downwind who will have to bear with the pollution, and who will never be able to match the war chests of a large company. This becomes particularly galling if that corporation is based in another country than America. (And no, there is no magical percentage of a company that must be owned by Americans before they have speech rights.) Why would Sony care about some plebes in East Overshoe?
And of course, as has been pointed out, no single individual has the financial resources to draw upon that a corporation does, yet they are somehow supposed to be equivalent in rights. Bill Gates is worth 50 Billion (or so.) There is only one Bill Gates. Corporate profits measure in the trillions every year. You do the math.