I think most on hear have been around bad contractors and new construction enough, then when they hear that a new sytem is only lating 8-10 years, it's not unreasonable to suggest that it wasn't installed correctly. IF that's true, the installation issue could be repeated with a direct replacement if the installer doesn't pay attention to details. Line size, ductwork, how it's charged, equipment sizing (don't assume the builder sized it correctly) all play a role is a good, reliable installation. You location is somewhat moderate in terms of cooling degree days compared to a southern state, texas and central states so there's little reason if properly maintained, that a unit shouldn't last closer to 15 years, even builder grade.
Thanks moto. Well thought out and detailed response (as always!) So what I've learned is to get multiple quotes, look for ACCA members, and have them pass my "test". This last part I could probably use some assistance on (if you're not all tired of me already). I wouldn't really know how to size them up. Any particular questions or behaviors I should be looking for?