That's a great question! I have not ever rented a car myself, since I was part of a tour during both visits.

Public transportation in Lisbon was great, as you might expect with any big city, and Portugal has a network of trains that connects the different cities.

However, if you are looking to visit some of the more remote areas (Sintra-Cascais Natural Park or the Fisherman's Trail) or smaller towns, you might want to explore the option of renting a car. From what I have heard, it is relatively easy. 

I hope this helps!