Sometimes I think people really expect too much. Remember the saying 'you get what you pay for'?
On my flight from McCarran back to Philadelphia there was nothing but groaning and complaining from the passengers. Yeah, it's a pain to pay extra for bags, but they're pretty upfront on their webpage that if you pay in advance you won't get hit with extra charges. I saw a guy completely miss the flight because he was arguing with the staff as to what constituted a carry on bag. I really think people get an attitude because this is a low cost carrier, and try to get away with things they wouldn't otherwise do on other airlines.
That being said I actually thought my flight was A-OK. The plane itself was clean and while the seats are a little more cramped than average they're not unbearable. Spirit definitely beats Frontier airlines when it comes to cheap flights, in my opinion. The seats are definitely more comfortable.
From hearing stories I was expecting this to be the worst flight of my life. It was actually relatively pleasant. Read over their webpage so you won't be surprised by any extra fees, and you'll be good to go. This isn't flying in luxury but if all you need is an uneventful flight then you shouldn't have a problem.