They are very convenient. I pick up my younger son from school about 3 days a week. If my wife isn't home, I have to bring my 2 younger daughters with me, so it's easy to just buckle them in there. Plus, on the days that my wife is home, and the girls don't come with me, I can toss my son's bike in there to bring to him if he rode the bus to school or got dropped off that morning.
The one we have even has extra space behind the seat for backpacks and jackets and such.