(1) It facilitates conversation. When E is up on the handlebars, we are seeing the same things. I can talk to her as I ride, and as she gets older she can talk to me about what she sees. I have many sweet memories of cruising around Austin with HP chatting about the trucks, taco joints (he remembers every place we've ever had a taco!), trash cans, and anything else we saw en route.
(2) It's easy to ride. I find I have more control and am better able to handle the bike with the weight up front rather than over the rear rack. I was worried about a steep learning curve when I started using the Yepp (and even practiced with weights in the seat before riding with HP), but it feels almost same as riding riding on my own.
(3) Parking's a breeze. Maneuvering a trailer into a bike rack without blocking the sidewalk or sticking out into the parking lot is a challenge. With the front seat the bike takes up the same amount of space as when I'm riding solo.
(4) It is simple to switch between bikes. This is a unique feature of the seat we use, the Yepp Mini. There is a release mechanism under the seat so we can switch the seat from my bike to Neil's with the push of the button. We each have an adapter (mine for a quill stem, his for a threadless). When we go places as a family E can ride with me there and Neil can take her home. Or if we were meeting up somewhere and then going our separate ways we could switch the seat to accommodate.
My only caveat to my enthusiastic recommendation of a front-mounted seat is that it is hard to find an American bike that works well with the design. The geometry of most bikes here makes it awkward to ride with the front seat--either your chest hits the seat (and therefore pushes your child's head forward), your knees hit the seat as you pedal, or both. I ended up buying a new bike that worked well with the Yepp (an Electra Ticino). That may seem extreme, but I had been wanting an upright commuter bike to replace my mountain bike for awhile; accommodating the front seat was the final push I needed to make the switch. In the end, we bike so much that it was worth it to me to be comfortable and not make compromises on the fit. Moral of the story: if you are considering buying a front seat, I recommend bringing your bike in and trying it out to make sure it is compatible before making the purchase.