I love Nina's. For one thing, I just can't imagine Cathedral Hill without a centrally-located coffee shop (similarly, I can't imagine Cathedral Hill without WA Frost across the street).
As such, the location is prime and crucial. The space inside is lovely (it feels old and cozy, as anything should in a 19th-century building).
It's great that they offer a wide variety of loose-leaf teas; I'm particularly fond of their monk's blend, as well as their green with ginger.
As of March 2011, Nina's started offering vegan baked goods, made by an outside vendor (Venus Foods). I've tried their apple cake and their chocolate cake and they're both delicious: moist, fluffy and just perfect with a cup of tea.
My only gripe (and it's a big one) is that they rarely keep the cake in stock. It sells out almost immediately and it takes them days to re-stock, which is a real shame.
Also, Nina's could improve by offering some hot vegan sandwiches (how about something with mock duck or tofu?).
Other than that, this is still a great place and St. Paul wouldn't be the same without it.