I just realized you were asking about a subway diet......
If you wanted to eat subway at every meal, and your primary concern is a balanced and nutritious diet, you'd probably need to alternate sandwiches- or at least take a multivitamin. But from experience (finals week), you will tire of Subway after the fourth or fifth day of it. I actually couldn't bring myself to look at Subway for a few weeks...
If you're considering the subway diet for weight loss, you should really just begin to count calories. I'm pretty sure I lost weight during a weeklong period where most of my meals consisted entirely of ice cream of the high calorie variety. Hell, I finished one of those ~1/2 gallon Edy's tubs in a day. At worst, I didn't gain weight. But the only explanation for this is my caloric balance- I worked out like a fiend. FWIW, an ice cream diet will severely impact your athletic performance.
Anyway, back to weight loss. IIRC, a lot of the 6-inch "6 grams of fat" subs clock in at something around 400 calories. I can't speak for you, but I don't find that particularly filling. One possibility is if you were to have more meals spread throughout the day- this would assist in weight loss. All weight loss diets seek to create caloric deficits- the subway diet is just another recipe for weight loss.