Interesting to see what'd happen to PR economically; it'd be the poorest state in the union, but I'm not sure by how much: I've seen anywhere from $19,000-$27,000 for per capita income rates. (If the higher, as according to the Wiki article Economy of Puerto Rico, that puts the per capita income within a couple hundred bucks of Mississippi).
Perhaps even more importantly, it would be the first state in the Union where a majority of the population- and a big majority, too- speaks Spanish natively, the language of government and the courts is Spanish, etc. I dunno how long it would stay like that. Canada has Québec, and it works, but really only because Québec is so populous- it has what, 20% of Canada's population? (And, of course, it keeps trying to leave). Puerto Rico represents about 1% of the national population; several Western states have higher Hispanophone populations.
It might be a watershed moment for Hispanics of all stripes, though, not just Puerto Ricans; even New Mexico is 46% Hispanic, but less than 30% of its people speak Spanish at home, and like the rest of the States, the de facto language of the public sphere- especially when it's interacting with the rest of the nation- is English. But symbolically, I think it would change things, because Hispanics wouldn't just be a populous minority in an Anglo country. They would, at least in Puerto Rico, be on equal footing.
It might not last. As I noted, Puerto Rican statehood is likely to mean that PR might become a retirement haven, especially if it uses tax breaks to attract wealthy retirees. (But maybe not; you wouldn't really want to move there if you don't know Spanish).
Also, are we gonna get a Puerto Rico state quarter? It has a territory quarter, I know.