Did it for over 6 years, used the seat on back type affair, very visible.
The Pros -
It is good for the environment, and gets them out of the car.
Gives the child confidence at speed.
Is lots of fun for them (Faster Daddy Faster!).
You can cut through parks and take scenic routes (although the 35mph downhill was their preferred route)
The Cons -
They get the occasional kick in the face as you get on/off the bike.
Slightly unstable, especially at lower speeds.
Most drivers don't give you extra room.
Fear of crashing/accidents, although none actually happened.
All my kids made it through, one even progressed to a trailer bike "and liked it when cars came close" (I didn't - it was very unstable when they moved around)
Main reason was their nursery was a 25/30 minute walk for their little legs and was on route to my work - 7 minutes by bike including loading up at the start.
If we all stopped riding on the road because of the behaviour of a few drivers they will win.