I own a Yuba Mundo and a Bakfiets-style bike.  I've been riding my wife and 30 lbs child around on it for more than a year.  It is a relatively affordable and flexible option for a cargo and family mover.  I was able to put some miles on the Surely Big Dummy as well and while I find it to be an enjoyable ride, I much prefer the Yuba as a practical transportation solution.  If you are going to load child carriers on the back of the Yuba avoid the full size panniers and opt for the shorter Boda Boda baguette bags.  Also worth noting is that while the rear pad is easy to work with it doesn't provide a tremendous amount of cushion.  Lastly, as load weights approach 150 lbs in the rear, the Yuba gets a little whippy and handling and cornering suffer.

My Bakfiets bike is the new Workcycles version.  It is far easier to maneuver under heavy load because of a lower center of gravity.  I opted for the long rear rack with cushion and foot pegs which now holds the wife and we have two little ones for the front.  As long as you are comfortable slowing down a bit and cruising (it handles like a early 70's Cadillac) the Bakfiets is a much better option for family hauling than long tail cargo bikes.  The kids love being up front due to the visibility and the rider/child interaction is far greater than having kids in the rear.  The cargo box is large enough to bring one kid to Costco and return with a cart of groceries.  Something I was able to due on the Yuba as well but with much less stability during the load/unload process.  And there was again the whippy factor.  Unfortunately, the luxury of a bakfiets style bike comes at a cost.  My bakfiets was over twice my Yuba set-up but the safety, frequent use and omission of a car made it worthwhile.

If you decide on the Yuba I have a black one in good condition I am willing to sell at a very reasonable price. :-)