I used to agree with the idea that corporations shouldn't have rights, but then I read a well-reasoned legal analysis of the Citizens United decision. In that, the lawyer pointed out that certain rights need to be granted to corporations in order to prevent the rights of individuals from being trampled on. One good, clear example of this is the Fourth Amendment.
If corporations don't have rights, then the government would be able to enter any corporate-owned building to search, at will, and without a warrant. The government could also seize any corporate-owned property (i.e. a laptop assigned to an employee under investigation), monitor any corporate-owned resource (all of AT&T's telephone wires, anyone?), etc.
The takings clause of the Fifth Amendment should apply, as well, otherwise the government could simply seize any corporate asset at will without compensating the corporation and, by extension, the shareholders.
First Amendment-wise, corporations have a certain amount of rights. Without these rights, the government would be able to censor what corporate-owned media could print/say, which WOULD be a direct infringement on the individual right of free speech of the writer.
Corporate personhood is fraught with peril, though. Corporations cannot be sent to prison. They cannot be executed, at least not in the conventional firing-line sense. They are, effectively, immortal, which means that they can do things like attempt to draw out lawsuits so long that the opposite party can't proceed since they're no longer alive. They have, by their very nature, the potential of having significantly greater resources at their disposal than any one individual. Because of these inequities, there really should be some form of legal restraint to balance the equation. Unfortunately, it would most likely take a Constitutional amendment to achieve such balance, which is almost certainly not going to happen, especially after Citizens United legalized unrestricted funding of Congress-critters.
So, while I disagree with the result of Citizens United, I can understand the logic that the SCOTUS used to reach that decision. Within the framework that they have to work with (i.e. the Constitution and SCOTUS precedent regarding corporate personhood), this was the only decision that could be made without attempting to draft law from the bench.