I'm not sure about Spirit, but my experience with other carriers (UA, NW, AA, DL) is that service has declined "somewhat" over the past several years. However, the variability is significant. Some flight crews are outstanding and very service oriented, while others perform the disappearing FA thing as quickly as possible.

Recent United flight from London to Chicago bore this out. Had to go and ask twice for another glass of water. The FAs were in the galley, but conversing.

Having said that, I've received GREAT service on many shorter flights.