For shorter flights, ACY to Orlando, or PHL to MCO when I am in need are just fine on Spirit. Prices are great when you can get round trip all in under $100. Are you in sardine class? Sure... but the upgrades to the Big Front Seat are pretty nice and cheap.
Having Spirit come to PHL, and expanding to other major airports is welcome news, as no doubt it will place competition on the legacy carriers and others.
Tell you what, no matter how much hate people give Spirit.... they are one of the more profitable airlines out there, with a stock price going up. lol.