I'll start off and proclaim my bias here. I don't like trailers. I think they are useful for cargo and out of rush hour leisure rides, but i really don't like them when your in town or crossing busy junctions.
My son agrees and still resolutely refuses to go in the trailer. He wants to be on the bike with daddy. In fact he wants to ride the bike and have daddy sit on the back. He's pretty convinced he knows how to do it :-)
That said they have a lot of benefits:
-Can carry a lot of 'stuff'
-Gives the kids some freedom to do things
-Safe in a crash that doesn't involve cars
-Can carry infants from 3months with suspension and inserts
-Easy to load
The disadvantages are:
-They 'feel' vulnerable in traffic
-Increased width on the road (suddenly i feel like I'm holding everyone up)
-Awkward to park
-Restricted view compared to bike seat.
I often read\hear people assure me that drivers behave very nicely round trailers and in my experience they do, but i feel that I'm holding up the traffic (its harder to pass on our narrow roads). In rush hour i cant imagine drivers would tolerate being held up like this.
I do recommend having a trailer. Its a nice add-on to a child seat combo and increases luggage capacity for days out (I can fit my birdy in mine!). I'm really hope that in the summer on long leisure rides in country roads i can persuade Mike to use it , just to get him out of the sun for a bit.