I have never commuted, but I have used my burley to deliver kids to preschool fairly regularly. My youngest is not overly fond of the burely and I went to the lbs to buy a seat for my bike and the owner told me that he would rather forego the sale and help me find a way to make my dd happier in the trailer. They are safer for a number of reasons:

1. You are less likely to fall when you have a child in a trailer than you are with a child on the back of your bike. the closer the child gets to the max weight of the child seat the bigger percentage of the overall weight of bike+you+them, and therefore the child's movements can affect your balance and cause a fall.

2. If you do fall your child is safer in a trailer since it is attached to the bike with a universal joint and has a low center of gravity - most likely, the trailer will stay upright.

3. If the trailer does flip (some of better safety records on this factor than others - do your homework) your child is surrounded by a safety cage. Always have your child strapped in (there is a 5 pt harness in most trailers) and always have him or her wear a helmet. Many trailer related injuries are attributable to parents bein over confident in the trailers safety and therefore not securing the child and not insisting on a helmet.

The one disturbing statistic is that injuries to kids in Burleys are often more severe than those sustained by kids in seats, and this is due to the fact that they often involve motor vehicles. You need to be extra aware when crossing streets and remember that it is going to take longer for the entire bike plus trailer to get through and intersection than it takes just you on your bike to get through. Also always have your trailer well marked with flags to make it more visible.

Finally, while trailers are more expensive than bike seats they retain their value quite well and your can get 50% or more back on your investment even after a number of years of usage.