Green Grotto is a pretty average place in my opinion. I've been here twice before, and both times were fine.
The bubble tea itself is pretty good, but slightly overpriced. I went with a mango slush and it was great. I went with a group of 6 both times and no one had complaints except for the prices. It's best to come here with a group and play card/board games at a table, as most of the customers were doing. I liked that they had free popcorn to snack on with your drink.
Overall, if you're closeby, really want bubble tea and willing to spend more than you should, Green Grotto is a safe bet. There are definitely other places around with better bubble tea for cheaper.