Upload & Sell: On
I've been using MOO.
They output to a glossy, heavy stock, allow you to break up an order with multiple images (no minimum required for one image), total control of front and back design, have fast turn-around, and are committed to correcting details you aren't happy with.
So far, the colors have been spot-on. The images, overall, are toward the darker side, with a tad too much contrast for my taste, but I still feel confident sending them out to clients.
No proofs prior to sending the job to the press
No corrections on their end, you have to re-submit the art after you adjust to your likeness (if you aren't pleased with initial run)
$1.00 a card, minimum order is 20