I've flown out of Mesa-Gateway Airport on Spirit Airlines to Dallas, TX for at least once a month since August and I have to say, Mesa Gateway is far better than Sky Harbor. Yes you have to pay for everything from bags to water, but the saying does apply for this airlines.. "You get what you pay for".
I never pay for my bags, i pack light and bring a small backpack that can hold 3-4 days worth of clothes plus toiletries (pants, shirts, socks, underware, bra's) most of these items can be worn more than once so it makes no sense to pay $35 a bag for both flights. I also never pick the seat i want to seat in, they give you an option to make the selection random and to be honest, i don't care where i seat, especially since i don't have to pay that way. I've flown enough to know it really does not matter where you sit, it's still going to be some what uncomfortable. The only real gripe i have for this airlines is the fact that the seats don't go back. My flights are only 2 hrs long but due to the time when the flights leave, (12:55 am) it would nice to be able to close my eyes for a little bit comfortably. I have invested in a Plane Pillow which works wonderfully no matter the seat I'm in.
All in all, if your cheap like me :) than you will appreciate this low cost airlines, especially paying between $75 - $150 for a round trip ticket (which is unheard of nowadays)
Hope this helps anyone out trying to decide if Spirit Airlines is the best way to go.
Shelly