I bought the Yuba Mundo from a place called Vancouver Long Bikes and had it shipped here. Couldn't be happier.
I ordered it sight-unseen, but it was all they promised. Very stable with big loads, and even though it is a bit slower, it feels much more comfortable than sportier bikes, probably owing to the fat tires and long wheelbase. Further, it is slower to accelerate due to the weight, but cruising speed is only a few kph less than a relaxed ride on a commuting bike with 700c wheels (only really noticeable if riding alongside one).
I looked into cargo bikes a fair bit on-line, and came to these conclusions:
The Surly Big Dummy is a "proper" bike - basically a long-tailed mountain bike made of good chromoly and well spec'd with parts. Lighter, sportier, but less of a workhorse.
The Yuba is built like a tank - very solid and heavy for real load-carrying. The most utilitarian of the long tails.
The Extracycle is an add-on, said to be less stable. Better if you already have a good donor bike and you won't be carrying too much weight.
The box bikes (bakefiets etc) are very good for cargo and children, but require a bit more space/garage and aren't the best for going up stairs and such.
An excellent source of info on this stuff is
http://www.joe-bike.com/cargo-bikes/, who are a retailer in Portland, I think. Also, while I have no interest in the business myself - the Yuba frames are on sale at the Yuba online shop
http://yubaride.com/yubashop/5-cargobikes for those that like to do their own builds for $350 (note, though, that the rear axle is larger than standard).
I like my cargo bike. I make excuses to buy heavy stuff at the grocery store!