I use an
OK Baby (very much like the Ergon model, but an older version) with my daughter (18 months, 10kg or so) and occasionally my son (4 years, 17kg). My son is at the upper end of the weight limit that I'd ride with, cause he shifts about. I agree totally with the line about balance being upset by a moving weight over the back axle. But a trailer just wasnt worthy for such a low-mileage option for us. If I had good roads/tracks and more time, maybe so.
My main problem (well, the kids problem really), is that I am tall, and sit VERY high on my bikes. Mounting the seat as directed, lower to the back wheel means they get a great view of my, ahem, lower back. My son is on training wheels, so he's pretty much skipped the tagalong option and gone straight to riding himself. I dont take my kids all that far on the bike. If I go alone, I might do 40-50km, but with them its down to the park, play around, go via the shops and home again, maybe 10km.