While I am not familiar with options outside of the United States, I have used Postagram for the iPhone. I found it quite handy while traveling to Europe this summer. I sent my family back a few postcards at $.99 each(they used to charge $1.99), and I also sent myself some back home.
They take about a week to process, but the quality is excellent. The biggest advantage I found was that I can do it very easily right from the iPhone app. Signup only took seconds, and once you enter in your credit card info once, they keep it on file for future purchases. I would have preferred that they used the Apple Store payment system so I didn't have to provide my credit card to a third party, but that is how they currently operate.
As far as print quality, I've only tried Postagram, but have found the quality to be quite nice. It is as good of quality as you can expect from a single print option like this. If you want to print 100 or 1000 post cards, you can get real offset printing that is quite a bit higher quality then this. But I do not know of a company that offers this for single postcards - due to cost and complexity.
Some examples of the Postagram service -
Take note, these were sent through the mail for 5 days, so they get pretty torn up!