In reply to EeeByGum: I love our wee ride and so does my son. I've been using it since he was about 8 months and he is only just about growing out of it now at 3.5yrs.
I use it on my xc full sus so there is a reasonable amount of room. Yes it is not ideal for your knees but mine are utterly screwed anyway so I don't care that much. For me the advantage is we can ride offroad downhills as it doesn't affect the centre of gravity too much, and he is looking ahead with me so we can talk about the birds, horses etc we see.
He doesn't need shielding, he just wears a wind proof, mitts and wellies though admittedly he doesn't seem to feel the cold. That said it had never occurred to me to do the nursery run in the rain, I use the car for that.
Finally, I couldn't actually use a rear seat as I only have a full sus and a cross bike so I guess it does depend on what you ride.