I've been having a great time creating Shells and Christmas ornaments for Shapeways!
So far, I've only gotten a couple of samples back (more to arrive soon!)
I've created these using Blender's Python api, which has been good fun to learn.
And then I've been using Meshlab to apply various color schemes.
(I'm working toward creating the color directly in blender, but Meshlab has given me some very nice results)
Originally I was just planning to create interesting Art and Sculptures, but then my wife gave me the brilliant idea to turn them into Christmas Ornaments.
As far as my experiences go, many of these have been pushing the limits of Shapeway's checking and processing.
My little programs can easily generate shapes that exceed the 1 million polygon limit. I've tried keeping them to around 800k, but even then, I sometimes don't get a 3d viewer or rendered example.. Netfabb seems to help some for fixing and repairing.
I've had great support from Martin, who has been very helpful and has given me some great tips.