I've ordered several cafepress shirts and the quality has been pretty darn good.
With artwork varying from simple text to full color designs with fades, they all seem to come out pretty well. They usually trim the transfer so you don't see much of the edges, and like most transfers, the edges usually dissappear completely after the first wash.
They are also using the new direct printing on a few of their products (to be launched on all of their products later). I've seen a few samples of the direct printing and the print quality is pretty close to screen printing or dye sublimation. There are a few color shifts, but definitely a step up from the heat transfer in my opinion.
They also stand behind what they sell, so if you don't like what you purchased, you can send it back for a refund or a reprint (because it's print on demand, you will get the odd misprint).
Part of it has to do with expectations I think. If you're expecting a screen printed t-shirt when you order from CafePress, you'll probably be dissappointed. If not, I think you'll dig the design and the printing.
I've sold through cafepress since 1999 and I've had very few returns (and lots of sales). If there was a quality problem overall, I would expect the return rate to be much higher.