Yes, Puerto Rico should become the 51st state.
In 1889 American gained territorial possession of Puerto Rico. In 1952 the people of Puerto Rico approved a constitution for themselves and they declared themselves to be a commonwealth. Commonwealth's enjoy more autonomy versus a unincorporated territory which does not share the same benefits.
Currently there are several characteristics that makes Puerto Rico part of our union of states. First, their government is set up like a state in the United States. They have 51 seats in the house of reps and 27 seats in the Senate. Their leader is an elected "Governor." Second, our federal court system is integrated in to the Puerto Rican legal system. Third, our military has an active and albeit horrid presence on the Vieques island (whether we should stop the bombing there should be another topic of debate). Fourth, the United States has control over the same areas as it does when it comes to the current states in the US. Under the 10th amendment, there is a balance of federalism. The US controls social security, immigration, and even the postal service in Puerto Rico.
The only difference currently is that this commonwealth is exempt from the IRS code and has no representation in Congress. As a result of its lack of official statehood it does not have allocated funds from the Government as other states do now.
There is no disadvantage, in the short term, there may be some budgetary fixes that would have to be made to allow for representation of Puerto Rico in the Senate and House of Representatives. There would also have to be a budgetary allowance alloted for them. In the long run, with the proper admission, there would not be much change in the status quo.
The concept of "territories" and overseas acquisitions is a very colonial and outdated ideology. Having countries like Puerto Rico, Guam, and the Virgin Islands incorporated as States make for us to have a larger presence (if you like that) and/or allows people to feel that they are worth more than just a prized possession.