A very low stress airport.
It's never very crowded, and you can always clear security pretty quickly. From here, you can easily get to any of the major hubs and be to wherever you need to be.
Even security here seems relatively laid back. At least at the checkpoints, it never feels like I'm dealing with the same poorly trained goons as at Milwaukee or Chicago.
Parking is easy (well, when there's not construction) and very cheap compared to most airports.
My only real complaint about MSN is that it's not an international airport. It would be nice to have direct routes to Toronto, at least. I suppose it's stayed regional because of the bureaucratic problems that going international would entail. It would also be nice to have more incoming flights from the major hubs each day, but given Madison's size that's certainly a pipe dream.