Hi Eric,
I had a great experience with my son on the rear mount "Co-Pilot" on my Clem.
It worked great for the first 1-2 years of riding with him, but as he got older and heavier around 2.5-3 yrs (he's 4..5 now) the disproportion of weight became a bit of a bother on hills heading back home, so I eventually went with a hodgepodge of used parts for a kit Xtracycle MB-2 build that I'm quite happy with for city riding. My son loves being on the back of it, and scoots up towards me and we chat along the way home from school through the park, and being a cargo bike it allows us to strap his push bike to it and we will ride together for certain stretches on the way.
My other son now a little over 1 year old has been using the Co-Pilot on my wife's bike, and I think we may end up eventually going for a Yepp mount on the Xtracycle so the boys can be together.
I agree with Lynne, bike trailers are a great option too. We use a trailer on weekends when exploring and not in the city which is great, reason being (for me) I found after a short trip to the grocery store not feeling all that comfortable having them being be so far behind the bike with the busy traffic, and distracted driving, especially at intersections as the bike lanes here in Boston are as inconsistent as the good drivers are.
I do however love the idea of a front mount seat, seems like in that set up they are more part of the experience and less of an observer! If we didn't already have the rear mount Co-Pilot seat, we probably would have gone that route for our youngest.
However you go about it, riding with kids is a real hoot!!!
-Peter in Boston