Airport transportation depends on where we are flying from. When flying out of BGR (which is 15 mins from home), we have an arrangement with some friends who travel several times a year, as do we. We do "back-up" for each other. If departure/arrival times are sane, we just take each other to the airport. If they are crazy, we have each other's car keys and will swap out cars. SO... let's say they have a normal-timed departure but will get back at midnight when they return. We'll take them to the airport for their departure. On their return day, we'll take their car to the airport in the early evening and park it in our "usual" location. When they get back, they just drive themselves home. Same if they have a stupidly early departure- they'll take themselves to the airport and we'll go find the car in the lot later in the morning and bring it home. Saves around $100 in parking for a week-long trip.
If we fly out of Portland (2 hour drive), we drive our own car. Last time DH and I flew, the car was acting up and at the airport it would have been parked in an open lot for a week- and it seemed the car issue was worse in the cold. We rented a car to drive to the airport, and that was cheaper than 2 bus tickets and then Uber/cab to the airport.
When we fly from Boston (4 hour drive), we either take a bus from Bangor to Logan OR we drive to Portsmouth, NH, and take the bus from there.
When flying out of PWM or BOS, I also have the option of staying overnight at my sister's and the driving to Portland or bussing to Boston for my flight. We do this when the flight departures are stupidly early or arrivals are exhaustingly late.