A quick comment from someone who actually builds these for people every once in a while
http://waltworks.blogspot.com/2012/10/sick-day-5-cargo-bike.html-They are called "Bakfiets" in Dutch which just means "box bike". Often called Long Johns in the states.
-They are VERY practical for carrying large items (ie, boxes).
-They take up a TON of space and are pretty expensive/heavy/complex as compared to, say, a bike trailer.
-You can get a box with seatbelts for your kids and ride with them in front of you/talk to them, which is awesome.
-There are 2 companies that build them in the US: CETMA and Metrofiets. Both are pretty expensive but this is a specialty item. I also build them occasionally for customers but it's a huge PITA to ship them so I generally will only do it for local folks (ie UT).
-For casual cyclists living somewhere with hills, they may or may not work - expect the unladen bike to weigh about 60 pounds. Some companies make electric assist versions but the price is going to go up significantly.
-They are a bit different to ride than a conventional bike but not hard to figure out for any competent cyclist.
If you're on a budget and just want to haul groceries, a bike trailer is a much cheaper and more practical alternative. If you need to haul kegs of beer, or big boxes of what have you, they are great.
-Walt