It depends on many factors.
Pho Tai is typically made of broth that boils beef bones, lemongrass among other herbs and spices. The broth is extremely high in sodium, depending on where you get it. Some places use MSG, which in moderation (like most things) isn't too terrible for you, but I would hardly call it healthy. People with high blood pressure or at risk of it, should not eat pho often or at all, depending on how bad their condition is, etc.
Besides being high in salt, you have the carbohydrate factor. This leans towards what type of foods your diet consists of, and what you are geared towards eating. For example, if you are on a low carb diet, pho would not be a choice for you. The noodles are made of refined white flour, which is highly processed offering very little nutritional value and a ton of carbohydrates.
The meat is beef- there are different parts of the cow they use in different variations of the dish. The main beef used is TAI, which is leaner, and considerably healthy for you to eat. They also offer tendon, tripe, brisket, flank, etc. Some of these additional choices contain much more fat. If you are on a low fat diet, the noodles may affect you slightly, depending on how strict you are, but you would definitely have to make leaner beef choices or even switch to GA, which is chicken (even healthier sans skin).
If you are on a low carb diet, Pho would not be what you seek. If you are on a low fat diet, you may need to go easy on the noodles and carefully choose your meat.
The herbs and vegetables in Pho are side products (basil, mint, bean sprouts, onions, and sometimes cilantro). Although good for you, they are not enough to constitute a serving of vegetables in ratio to the carbohydrates and meat in- take (in a normal serving of Pho).
Which leads me to the last point, what serving size you eat matters. I've been to some Pho restaurants where even the smallest bowl size has been too much for me.
On a side note, if you add Siracha or Hoison sauce, be aware that both have a decent amount of sugar content (for those who are doing no to low sugar as in South Beach).
Generally speaking if you get the entire bowl of Pho with ALL the fixings, I would say it's not healthy. Eating a bowl of Pho with ALL the fixing (the fatty meats) in combination will give you a high fat, high carb, high sodium bowl of food.
If you are craving Pho- eat about HALF of what a restaurant offers you in a bowl and skip the fatty meats.