In short
Old San Juan is the oldest European settlement still inhabited under American sovereignty. Its two fortresses, El Morro and San Cristóbal, are World Heritage sites.
The island is small: the El Yunque rainforest, the bioluminescent bay at Vieques and the west-coast beaches are all within three hours.
What it really costs
Puerto Rico uses the dollar and charges American prices, which makes it one of the most expensive Caribbean islands for a European.
In exchange, there are no customs formalities arriving from the United States, the phone network is American, and the infrastructure works.
What nobody tells you
An ESTA is compulsory, as for the mainland United States. Plenty of travellers overlook it thinking they are going to the Caribbean, and boarding is refused without one.
The electricity grid has been fragile since Hurricane Maria in 2017: cuts are frequent, and serious accommodation has a generator. It is worth checking when you book.
A car is necessary outside San Juan: public transport does not serve the interior or the western beaches.