I just made my first trip to Cuba, speaking at a conference/advanced course for students at the University of Havana. There is much research activity in applied mathematics, and the involvement of students seems as advanced as in the US.
Cuba is now an easy place to visit, with inexpensive nonstop flights from New York and elsewhere and a visa readily available en route by checking a box.
There are some special difficulties for Americans. Dollars are penalized 10%. American credit and debit cards don't work. Western Union cannot send money to Americans; I had to have my mom send money to a Cuban friend for me. It's best to bring lots of Euros.
Americans with institutional gmail accounts can't even use webmail, though MacMail worked for me on the national net, available in hotel lobbies and parks if you can manage to buy a pass, at about $1.50 an hour.