As part of Silicon Valley Education Foundations iHub program, a co-worker and I piloted Makers Empire in the fall. It's a 3D printing app we used on our iPads. I haven't used any other 3D printing apps so I can't make a comparison, but I've been very impressed with it.
With minimal guidance (a 5 minute basic intro from me) my students were easily able to create objects to print. We primarily used the Shaper and Blocker sections of the app, although students have had fun with the Toy Tester. Creation involves easy drag and drop features along with re-sizing and changing colors of items.
After some initial play time, the first assignment was to create a monster to go with our "I'm Not Scared" literature unit. That was followed by creating landforms (social studies), a symbol of hope, a healthy snack (science) and a plant (science).
The teacher dashboard allows me to check and comment on student creations as well as send them to the printer. In addition, there's a lesson plan section with lesson ideas for specific grades and tied to Common Core standards. My students did one of the suggested 2nd grade lessons, which was to create a device that would allow Ormie (a pig) to get a cookie jar off the top of a refrigerator.
We plan to use Makers Empire for the rest of the year and even my co-worker's kindergarten students have independently created spiders and people.