On my first trip since I moved to Pennsylvania, I parked at the long-term parking lot at the airport. This was almost a disaster-- it took me forever to find a spot and I ended up running to get to security, where the line was blessedly short and I made my flight. After that experience, I searched Yelp for an offsite lot for my next trip, and happened upon The Parking spot.
I liked that all of the spots were covered (no snow, less bird poop), and that you can make a reservation, even if I didn't end up needing it. The shuttle drivers I had on both ends of the trip were pleasant. The price is very reasonable, especially if you find an online coupon-- I paid much less to park here than at the airport itself. I didn't have to wait for the shuttle at all on the way to the airport, but it did take quite a while for it to pick me up on the way back. It's a bit far from the airport so plan accordingly. On the other hand, it probably takes even longer to walk to the far reaches of long-term, so it likely doesn't extend your trip too much. I liked getting a free bottle of water as I left the airport.
I think they have a loyalty program, and I'll probably join if I travel much.