Most men wear jacket and tie but it is not required. I've never worn a tie and half the time don't have a jacket and did not feel unwelcome. Jeans would probably be a problem and T-shirt is likely to get you banned. Most are formal fancy restaurants and some guests are there for a special occasion "meal of a lifetime", particularly at Gaddi's. You can wear a tux to any of these and not feel out of place. For Felix women cannot have too much glitter, that seems to be de rigeur there (again not a dress code, but natural selection, and it is more bar than restaurant).