Very crowded in the summer mainly because of the 100 or so beaches, which include some of the best in Greece.
Most of the beaches on Skiathos are big and sandy with sunbeds but there are also nicer, less crowded ones. However, the island does have a lot more to offer than just its beaches. It is quite small and has a nice atmosphere, a lovely old town, and some excellent walking through shady pine forests. The frequent buses make it quite easy to explore a lot of the island on foot. And it has an airport. Much of the film Mama Mia was filmed on the island in 2007.