The really hard part for the consumer is finding a good company; they are out there, but I personally believe they are out numbered by the hacks out there. I started out by first looking at the BBB web site, which I'm not sure is reliable any more, but it has info on companies and customer complaints, and if they have been resolved. Then I checked with the my county's department of consumer affairs. I am not a member of Angie's list, so I did not check there. I found a good company, but far from perfect. To their credit, they did have one of their service tech's come out and address and clean up any issues I complained about, which were mostly cosmetic. It is mind boggling to have to go through this and having to spend SO much money. I just got a new central AC system installed last month. I chose Trane equipment, as this company sells Trane and York, but the Trane had a higher SEER rating and gets a much bigger rebate from our local power company here.
Make sure whoever comes out does a manual J and Manual D to measure all the room sizes, heights, number, size and types of windows, doors and their exposure directions. The Salesman that got my order spent more than 2 hours at my house, and most of that was spent going through my house with me and taking all these measurements to plug into the computer and determing the heating and cooling losses to decide which size and system and ducting was correct for my house.