The park is quite big and offers nice views of Barcelona. The monumental zone is overpriced (€7/€8) and quite small. My advice: do not visit the monumental zone, but only the free areas. If you do want to visit the monumental zone, buy your tickets online, as availability is very limited (restricted to 400 entering visitors per half an hour). You do not need to print your ticket.
The park is easily reachable by the TMB 24 bus service, departs from Pl Catalunya very regularly ('El Corte Ingles' side).