Spirit is fine when everything goes as planned. The nickel and diming is what it is. It's on the travelers to do their own due diligence and find out what all the assorted fees will be. However, when there are irregular ops due to weather, crews timing out, etc, that's when things can go south, and in a hurry.
For us, it was Spirit's complete lack of information/help we experienced when we experienced delays on both directions of a trip. Our flight from BOS to MYR was cancelled in between the time I checked online at home, and the time we arrived at the airport (~1 hr). We were delayed nearly 10 hours, our non-stop turned into a 1 stop. Fortunately, we did get out the same day. Coming home on this same trip, there were massive delays up and down the East Coast due to fog. WHen we left for the airport, everything showed on time. We went through security at MYR, and proceed to WAIT. Our flight kept showing delays, delays delays, there was no one at the gate to ask questions of, and no one answered the phone when we called. We finally took off (again) about 10 hrs late. The capper to this trip (and this has NOTHING to do with Spirit) is that we came very close to having to divert to another airport, due to low fuel, because we were delayed in the air due to fog in Boston. For those of you in the Northeast, we were doing big ovals right around Providence.
Our experience was about 10 years ago at this point, and I've still not ever flown with Spirit since. I get that 'stuff' happens, but for us, it's how the company deals with it, that makes or breaks the experience. We flew with Delta the following year, and were caught up in a massive snow storm. It took them 3 days to get us out, because we were flying over the busiest week out of the Northeast, and they could only get us into an airport 2 hours away from where we had booked. But, they were apologetic, and booked us in 1st class.