Cruise #1, Packed a suit - took up more than its share of room in the suit case, wore the suit to Lumiers, took off coat, ate dinner, put coat on took it to the room, packed it away - should have just chucked it overboard so I would not have to bring it back home.
Decided on Cruise #2 to skip the suit, and guess what, the ship didn't sink or anything.
So nope, I'm not "dressing up" for formal and semi formal nights. Wearing slacks, dress shirt and a tie. And heaven forbid, I'm not wearing dress shoes (see the suit above) - going to wear sandals. Sorry about totally ruining your cruise, but that is as uncomfortable as I am going to be on this vacation.
The rest of the evenings I'll be wearing dockers and polo shirts, except for Mexacalli... night when I'll be wearing my tropical shirt.
On our first cruise some of our table mates came to dinner in jeans and tee shirts. Was that under dressed - yep, did it ruin my dinning experience - nope not at all.
There are always going to be people dressed better than you or worse than you. For me, my dress up is going to be what the tux wearers will be wearing on the more casual nights. Heck, if I brought a tux on the cruise, I would probably wear it every night except for the Mex/Tropically... nights. I agree with the guidlines about not wearing jeans, tee shirts, cut offs, swimming suits in to the dinners (all of the dinners not just the formal ones).
You can't control what others will be wearing, you can control how you let it effect you. Going in you know there will be some people not following the dress code - so be prepared for it - and don't let it ruin your vacation. Remember that your food and service will both still be excellent.