We are very glad to have purchased a (48 hours) Lisbon Card, which enable us to use public transport practically unlimited (tram, metro, elevator/funicular, train between Lisbon and Sintra/Cascais, bus), which is very practical if you plan to visit a number of Lisbon's attractions.
You also don't have the hassle of buying tickets for the tram, bus, metro or train, which often quite time consuming due to long line at the ticket machine (e.g. for trains).
And with Lisbon Card, you can visit quite a large number of these attraction free of charge (basically almost all of the important ones).
So for us, a Lisbon card is the perfect solution if you plan to explore Lisbon quite extensively, saving time and money.