how does spirit air fly so cheap?

Their fares are so cheap because the pack their planes as full as they can. They have the least legroom of all domestic airlines. Go to Seatguru.com and click on one of Spirit's planes. All of the seats will be red (which means it is a bad seat). Like you said, they also charge for EVERYTHING. When you add up all of the additional costs, it isn't much cheaper than any other airline. With other airlines, your original fare is more expensive, but that fare includes a lot of the stuff Spirit charges for.

I'm very weary of flying on Spirit, it makes me think that if their fares are so low that they may be cutting some corners.

There are several reasons for this. First of all, their base fares are cheaper than other airlines, but they charge you for several things that other airlines don't charge you for (and some of these fees are quite steep, and some of them are pretty hard or impossible to avoid paying). For example, most airlines charge you to check bags on domestic flights, but Spirit Airlines charges you for carry-on bags as well (anything that can't fit under your seat). It's usually easy to avoid checking a bag, but if you're going somewhere for longer than a day, it's pretty much impossible to avoid bringing a carry-on bag small enough to fit under the seat. These fees are at minimum $26 each way per bag, depending on when you pay the fee. So this will cost you an extra $52 round trip (at least) to bring a carry-on bag going in the overhead bin. They also charge you a hefty fee if you print your boarding pass at the airport (no other airline charges that fee). If you want to pick your own assigned seat, there's another fee. They charge for non-alcoholic drinks, so even a can of Diet Coke will cost you $1 or 2. So by the time you consider the total price of your ticket with all these extra fees, they're usually not a lot cheaper than other airlines and sometimes more expensive.

Another reason why they're cheaper is because they cut back in customer service and other operational costs. They're often understaffed at the airports they fly out of, and many of their employees do multiple jobs.

One other disadvantage of flying Spirit Airlines is they fly out of a limited number of airports, and they don't have agreements with other airlines to deal with delays or cancellations. So if they cancel a flight, they can only put you on another Spirit Airlines flight - whereas many other airlines can put you on another airline if that will get you to your destination.

Well actually when you add in all of these fees they are actually not that much cheaper than other more main stream airlines. And if you get stuck with some of their fees, such as a Gate Check Carry On($100) they may actually end up being higher.

However, they also cut corners in other areas, such as Customer Service. They are constantly one of the lowest rated airlines. They also pack more seats in the plane, because of this the seats do not recline. While I can not confirm this, I have seen where they also pay their employees much less than the average of other airlines.