Despite living 1-2 miles from Giannotto's for years, and filing up my truck across the street weekly, I just tried them this year. I'm glad I did. I've been 5 times since.
David Giannotto, the owner, is very personable. He clearly cares about his restaurant, his food, and his customers. I've never left without receiving a heartfelt "thank you". Neat guy.
The food is a welcome respite for a transplanted mainlander. On Maui quality food can be tough to find if you don't want to spend $50/person in one of the tourist areas. The local places tend to have freshness and/or preparation issues. The menu at Giannottos is typical of many of the pizza places dotting the east coast of the US. Pizzas, subs, salads, calzones, pastas, etc.
The crust is thin/east coast style. It's authentic, and prepared correctly. All pizzas are the same size (~16") and feed 2-4 adults, depending on appetite. The veggie pizza and deluxe are excellent.
I have not tried any of the pastas yet, but salads are excellent. The greek salad is a basic interpretation (iceberg/romaine, olives, feta, onions, etc) but something about it is addictive. It's meant for 2-3 people, but I find myself unable to not finish it. Give it a try.
My only complaint is the price. It's a tad high - It's tough to not spend $40 on a pizza and a couple sides. The cost of keeping the pizza ovens at temp given Maui's sky high energy prices may be to blame. Pizza by the slice is also available for those on a budget.
There's seating outside, and a few tables inside if you wish to eat on-site. Main street isn't particularly scenic, but the temperatures at night are always perfect 65-70F. Giannotto's is BYOB. Alcohol is available across the street at Chevron, and they actually have a decent selection of wines and beers for a gas station ($5-$30).