Just watched their video on youtube last night and was going to ask the same question. I would assume that if you were already going to (or already do) use Quickbooks, this would be a huge time saver. Particularly if you were just starting a new business, or switching over to a new book keeping system.
Features I saw that looked more appealing:
Vendor list is already populated with the most common businesses
"Parts list" for the common t-shirt brands/sizes/etc
Sales order/Quote sheet that included an image of the shirt, description, price break down and spot for customer signature.
Let's say you pay an employee $10/hr. Could they really do all this for you in <15 hours? Probably not. And the add-on is only $150.
On one hand there is no limit to the amount of money that you can spend to "save money" or "save time". On the other hand, time is money and an minute you spend formating an excel spreadsheet or enter vendor info is a minute not spent with a customer or running a press...