It costs 7£, so it depends how long will you stay in Portugal, what you’re planning to see (museums, art galleries), how you’re planning to travel (train, bus, plain?).
I would say that if you want to see a lot of stuff, take it! But don’t expect a lot of discounts in local buses. Main discounts are to see cultural stuff – Museum, galleries, and concerts.
Also if you’re planning to travel anywhere else this year it could be advantageous.