I've flown Spirit 3 times so far. For the price of the fare, couldn't beat it. I paid $61 for one flight. What a deal!
Overall, it's pretty much you get what you pay for. No thrills, no extras, no nothing. There is Spirit Plus, the so called "business" class, but I have never flown that, so I can't comment.
If you get a cheap fare, then I definelty recommend flying Spirit.
Hope this helps, if you have any more questions, ask away!