I have flown Spirit three times, all three of them were between MSP and ORD. Two flights were last-minute so their price was substantially cheaper than any other option and the third time was booked in advance but between better price and better connection, it was the most logical choice.

All three times I just had carry on that would fit under my seat. I boarded at the very end of the boarding process (why sit in those cramped seats any longer than necessary?) and ended up putting my small day pack in the overhead compartment for no extra charge.