This can be a hot topic, depending who you talk to. I seen everything from jeans and tee's to ball room gowns and tuxedo's worn. The idea with Formal and semi-formal is to have a night when people can dress up and go fancy. But it's more "optional" than anything else. Granted most people do at least put a bit more effort into the clothing those nights and many goes full out.
Personally unfortunately this time the Tux will be left at home, and I might or might get my suite with me, might just end up with dress pants, nice shirt, tie and a dress jacket. We are spending almost 3 weeks after our cruise over in Europe with rental car and domestic flights so have to watch packing space very carefully. Normally since we fly Southwest inside the US we often have a extra spare bag with us so can fit more than enough stuff and we bring Tuxedos and Suite for me, fancy gown/dresses for my DW and full pirates costumes for the kids and some light pirate attire for me and DW. But not this time, have to watch every little thing that goes in our suitcases so most things that are a "one off" use will probably have to stay behind.