I own tons of shirts from Threadless, and all of them have been really great quality! One thing to pay attention to, though, is that their designs are often printed on a variety of shirt types (even when you think you're just getting a regular t-shirt.) They often say what type of shirt it is (slim fit, regular, etc.), so pay attention to it when you're picking a size. For example, for regular "girly" shirts I can get away with a Large, but if it's the slim fit I need to go up a size.
I've also never had an issue with shipping or processing time. They often run great promotions so I often wait on those to shop (unless I see a shirt that I just HAVE to have.)