Not bad and not great.
This place has been open for a long time, and it used to be packed. However, there was only two other tables when I visited on Saturday night.
I would say the pho here is very average, with considerable amount of MSG. I get really thirsty afterwards. Most people order the tomato beef brisket pho - it's actually pretty good. The beef is tender and the soup base is filled with flavour.
Overall, not much to say about Top Saigon. It's in a hidden corner on the second floor of Time Square. I'd say there's a few other restaurants in this plaza that are comparably better.