Tons of hidden fees!!! However, you get what you pay for. Spirit Airlines is reasonable if you are going on a short trip & are packing very lightly, such as from San Diego to Las Vegas.
About the hidden fees... Spirit will charge you for a carry on &/or checked bag & the price will increase at the airport if you opt out of purchasing it beforehand. If you are planning to pack a ton, do not fly with Spirit because in the end, it will equal the cost of an expensive plane ticket that includes a carry on/checked bag! If you can fit everything into what they consider a "personal item" which is a bag that is 16 x 14 x 12, then you are golden!
Also, make sure you print your boarding pass beforehand because of you print it out at the airport, it is like $5! What a jip!