I fly them a lot out of atlantic city and believe I get a good value for my money. The non value carriers have gotten me to my destination 36 to 48 hours late on many occasions, same day every time with spirit. Most of the bad comments on this board are people regurgitating things they read elsewhere and aren't first hand accounts.
I do have a firsthand account.....Flew Spirit once, and will try to not fly them again. We were flying from BOS to MYR for a long weekend. I checked the status of DS & my flight before we left our house, no problems, on time. By the time we got to Logan Airport (45 minutes later) our flight had been CANCELLED, we went from having an early morning non stop, which would have had us to Myrtle Beach by 11:30, to spending 7+ hours at the airport, and getting on a 1 stop flight, which did not get us into Mrytle Beach until 7:30 that night. OK, stuff happens, I did appreciate them getting us out the same day.
Coming home, again, checked flight status before we left our condo, all things as normal. Get to MYR, check in, and go to the gate. This day, there was heavy fog all up and down the East Coast, and our flight kept being delayed, delayed, delayed. However, there was NO ONE from Spirit at the gate to help us, answer questions, give updates, nothing. Turns out, our plane was coming from Boston, which was on a full ground stop due to fog, and the plane did not even take off from Boston until 2 hours
after we were supposed to take off from MYR. Again, I completely understand that weather is outside of ANYONE'S control, it was the complete lack of availability of anyone from Spirit to answer any questions, and the utter disdain that we were shown by the phone reps, when we finally managed to get through to someone. I did write them an email after the fact, and did not even get the courtesy of a canned reply acknowledging that they received the note.
This was several years ago, and knowing people who have flown them in the interim, I know it has not improved.
2 winters ago, I booked a flight from Boston to Atlantic City for my boss for a trade show in February. I did so under duress

and told him that I would not be held responsible if things went awry. Well, it did snow the day he was supposed to leave, and Spirit could not (would not?) get him out until the day after the show ended. Fortunately, it was an incredibly cheap fare, so he was not out a ton of cash, but it underscored why I don't like to use Spirit.