My eldest came out a few times in a child seat. He was fine and enjoyed it when awake or asleep but if he woke up mid ride he could get upset - not good when about 30mins from home. When number two came along 18 months later rather than try to take two kids I decided it was better to spend quality time playing with the kids and quality time riding my bike on my own rather than trying to mix the two. Subsequently my youngest has never been in the child seat.
I have a Burley trailer I was given which for the same reasons remains unused. I have however rented one when in Aviemore and it was a barrel of fun for the whole family because of the lovely trail network and Lochs to stop at (Cheers to Bothy Bikes).
I think it depends where you live and how you want to ride. I was never happy with the kids so exposed on the road, and any offroad around here is either tame fireroad or scary singletrack. Tame fireroad would be boring for the boys as it's all in the forest with no views and nowhere to stop.
With hindsight I would not have bought the seat or accepted the free trailer which clogs my garage.
Recently tried the eldest (3-1/2) on a tag-along at Aviemore and I think his mouth hurt almost as much as mine from all the grinning - it was brilliant. Bothy bikes had a double tag-along which I look forward to hiring next year. Not sure I want one here where I live but for holiday it was ace.
I think my seat is a Hamax Sleepy - it was great when we used it.