Piedmont was a good airline. The other carriers that became US I never flew.
In my opinion, based on my flying from 1997-2005, pre-merger US Airways had a good product, but a very poor balance sheet, and 9/11 made it worse.
I never flew America West, but those here in this forum who flew them seemed happy with its product.
When the two different cultures came together, they clashed, and the two as one became bad. But it didn't have to be that way. The new US did not have to get worse before it got better.