In reply to Tommy Harris:
I never used a rear-seat, but had our first child on a WeeRide which was great.
There's a big caveat that the comfort of a front-seat depends on your size, and your bikes geometry as to whether or not it's comfortable to ride or not. I only used it for nursery runs and the occasional lap around the local woods - and whilst I was a little bow-legged, it was generally fine.
I liked being able to have a nice chat, and I/she felt that much safer as were we to crash, you're already kind of protecting them. This is probably more psychologically than physically true.
Also if you have full-suspension, they benefit from it.
My only slight complaint was that on my Canondale, which has a silly-big headset it's chipped the paint slightly as it was a tough fit.
With the addition of child #2, we ended up with a trailer they both fit in which is great for longer rides - but it's hard to chat to them. They however just chat to themselves. Not cheap, as got the Burley D'lite specifically as it has suspension so is way more comfortable off-road. Also means you can take it off (in about 30 seconds) to have a normal bike again.