Great for vegans because they have a vegan cheese option--and it's good, too! Very rare and welcome.
Most of their meat sandwiches can be made vegetarian--they use well-prepaired seitan (wheat gluten) instead of the meat.
Not so good for gluten-free... too bad. They could offer a gluten-free bread choice. They do at least let you bring your own gluten-free bread for them to use, but that seems like a halfway solution.
The food is very good, but I only go occasionally when I'm really hungry! It's terrible for you! Lots of carbs, lots of fat. I love it! But not all the time. Maybe they have healthy options, but that's not why I go to Melt. I go for huge greasy vegetarian sandwiches and great seasoned fries.
They lose a star because the wait can be surprisingly long, and because you sometimes get packed in elbow-to-elbow with strangers, and because parking is very limited, and requires quarters. Go at off hours to avoid these problems. I have not had a problem once seated: service has been fine.