I tried Spirit down to Costa Rica last year... hated them. You have to be among the first to board to have any shot at all for overhead storage space, and the seats (which are charged extra on top of the ticket price) are so tiny. Not my cup of tea.
Yep, that's a unmentioned feature of the A320 series - they made their overhead bins just a tad too small. Spending lots of time flying United on their A320s and 737s, it became quite clear really fast which one I preferred. The standard sized carry on bags which fit fine on the Boeing, are an inch too long for the Airbus overhead bins, which leads to people loading them sideways and taking up more space. That leads to extra long boarding times as half the passengers now have to check their carry ons. Really smart engineering there, eh.