This is an update to my first review from two years ago. They've improved customer service and reliability considerably in the interim.
I uploaded my artwork, selected my shirt colors and sizes, and entered my credit card information. I got an acknowledgment within minutes, and a follow-up proof I approved in a couple of hours. I got a phone call from a staff member who checked to see they had the sizes I wanted in stock (I ordered a M, L, and a 3XL), and when he confirmed they did, he told me they'd have it ready the next morning.
Pretty great. Easy interaction and fast turnaround. They've improved wonderfully.