My 2p...
The ceramic ball is designed to be lighter than a steel ball, which allows a ball bearing to rotate at a higher speed due to reduced centrifugal loads, and hence reduced skidding at high speed. However, for a differential, that's not what we need. Fortunately, they also have one property we do need - they are very hard compared to steel balls, so last a lot longer.
Smokes, the method of manufacture is the same as steel and carbide balls, so ceramic balls are no rounder than any other ball bearing.
As for building and maintenance, there is no difference whatever the ball type used. Once the rings are sanded flat, as Smokes says, then make sure they are roughed up with 600 wet 'n dry. This gives better grip between the balls and the rings, allowing less pressure on the end screw to get a slip-free diff. The secrets of diff-building are here...
http://www.teamirsrc.com/techtips.html
...from the Diff Dude himself, Dave Irrgang. It might be a 12th diff, but it works on all diffs.
The secret to long life between rebuild is keeping the dust out, and making sure that the diff never slips. As soon as the diff slips, the balls score the plates and make a groove, the tension is lost, and the diff slips more. Keep the grease to a minimum, keep the dust out, and keep the diff tight. Whatever balls you use, follow those guidelines and your diff will go ages between rebuilds. HTH
