Kim and Rob from Jet Set Life still had CafePress shop on their site a couple of months ago. They sold all sorts of stuff there, from branded tee mugs to clothes. The shop has now disappeared, so my guess is it wasn't a success to them. (Or maybe I just can't find the shop on their site anymore..)

Anyway, I think the success of your shop has a lot to do with your site's target audience and the "hipness" of you're (possibly) branded products. If you've established yourself a solid and respected brand among your audience, some people will probably buy. But I think the shop + products sold there has got to be a natural part of the site you're selling them from. Some random t-shirt shop in a, let's say a mobile tech blogging site just doens't make much sense (unless it's a Engadget and they sell some really "cool" shirts that make all the nerdy ones drool..). But that's another story!

You should also expect to generate traffic from your own site and not rely on Cafe Press' Marketplace. They used to allow you to set your markup for their marketplace too. Now they set it themselves very low. But if you can generate traffic to your own site, then that is not such an issue.