There are many transportation services that will get you to Portland airport. Portland has public transportation like max and light rails systems that are setup to help keep traffic down in the city.
-- Yes, taking a cab can be the fastest and most convenient way of traveling from an airport to your accommodation, or for taking you home safely at night. A taxi driver is to meet you at the agreed pick up point. It's like having a friend meet you at the airport. He is there even if your plane is delayed, knows where you are going to and takes you there. I always use taxi transfers...