I've been here a handful of times. The building itself is really nice and makes for a great spot to relax or even bring your laptop (free wifi!). It can get a bit busy in the evening (after 6 PM) but I usually go around noon to 3 PM and it's easy to get a table.
The taro hot milk frothie was a letdown for sure - it came out lukewarm and ended up being pretty meh. On the other hand, the cold rose floral tea is probably my favourite drink from Green Grotto. It's light, refreshing, and the subtle rose flavour is delicious.
Prices are a bit higher than other bubble tea places, but they do have a wide selection that includes some pretty interesting and unique flavours.