The answer is, yes, if you ate three subway sandwiches a day (one for each meal) and nothing else, you would probably lose weight. But the concerns brought up by CrunkChipmunk, MsGhettoBooty and maleficent are very real.
Advantages: convenient, tasty if you like Subway, straightforward (i.e. not many food decisions to make).
Disadvantages: very expensive, low variety of nutrients (especially not enough fruits and vegetables), can get boring after a while, relies on having a Subway near you at all mealtimes.
In the end, it's a question of how this diet suits your particular situation.