I've noticed that all of the reviews on Open Rice for this shop are in Chinese, except one, which is partially in Chinese, and clearly from a local HK Chinese.
Perhaps local tastes differ.
As a foreigner with a taste for both freshness and flavor, I can say that among the dozen or so casual Vietnamese restaurants I've tried in Hong Kong, Cadogan Pho is consistently the best. Their beef Pho has a superb flavor, accented as it should be, by plenty of basil and fresh onion, in addition to decent beef (best taken raw/rare and allowed to warm in the soup). Salads are superb.
Their menu is extensive, offering plenty of salads (recommend the green papaya salad as well as the pomelo salad), snacks and dozens of kinds of noodle choices, all at reasonable prices (not super cheap and by no means not expensive).
I can't eat at this shop without ordering the delicious and beautiful shrimp rice paper rolls. Our other, new, local Vietnamese restaurant doesn't even have this on the menu.
Unlike some other casual Viet/Thai restaurants, the lime soda tastes fresh, not too sweet, and not at all artificial.
Tables are standard HK noodle shop style with foldable stools. For a tiny shop, this is acceptable.
Service is very good, always with a smile.
Only problem I have is that as as internet addict, I'm annoyed by the low cell phone reception and no wi-fi in the shop.