For whatever reason, Artscow.com seems to have become the standard bearer for print-it-on-your-own card games and decks on boardgamegeek. Even having the need to use Artscow specifically for a few contests people are running there. I'm not really sure why this is; I've had reports of other places doing it cheaper, or more efficiently, or whatever. It's not that they are bad or anything. They have rotating daily (or even hourly) specials, so that helps. Also, they have a pretty nice interface for uploading cards through the use of Silverlight.
It is the first place I've seen that let's you create a deck cards where you can specify different card backs for each card, which is exceptionally nice when you are doing odd ball deck sizes, or smaller, differently handled decks (as such that I was playing with).
Anyway, the biggest issues I've found are with the fact that cutlines supplied with the image importer don't really correspond nicely to the actual cutting edges of the cards. Looking at the top row of cards in the lousy photograph over here to the left, given enough whitespace along the edges, and they look great.
However, bleeding out the images to full size is less-than-stellar, due to the over-growth of the images in relation to the physical cut lines. The text on the "Oracle" card even got chopped a bit off the top. The other two cards have a second ornamental bezel applied around the edge, which got severely cut (you can see traces of the extra bezel in the corners.
Otherwise, the cards are a bit flimsier than your standard poker deck; I am not well-versed enough in printing productions to know if this is because of the plastic coating or paper thickness.
The actual edge cuts are nice and crisp; even though I know of someone else who had small complaints about the cutting.
As far as prototyping goes, they are perfectly fine and feel and look "real" enough to fool your friends and family.