They look quite complex but are really pretty easy to make. The toughest part is the sanding.
These bowls can look incredible when cut from a nice hardwood with lots of figure. You can also do glue-ups of different species to make patterns in the bowl.
Sometime around 2009 or 2010 Carole Rothman published a book taking the technique to a whole new level. In her book she has designs of several different bowls. If you are interested in this technique you should certainly find her book. Here is a link to her book on Amazon.
DVD and the 2013 update DVD at this link. DVD #1 is $20 and contains over 1,100 patterns. The 2013 update DVD is $7.50 and has another 178 patterns. The 2014 update DVD is $7.50 and has another 223 patterns.