I think it's relative to where you are flying from. We have found flights from Pittsburgh and Buffalo (we live in between) to MCO via Southwest for as low as $178 RT. The most I would pay to MCO is $250 PP. We have SW miles though, so it's a bonus to us to just be able to rebook at a lower price and get those miles back (or travel credit, if we booked with a CC).
I have heard things about Spirit, most friends I've spoken with said it was fine and it did the job, but complained about all of the extra fees. If the deal was really good, I'd probably give it a try, but I'm also probably willing to pay a bit extra for the comfort/ease of SW.