Getting one of these helped change my life. I used it first with my son when he was about 18 months, and then once dd was big enough, I used it with both...until I added the tag-along, and then I cranked it for a summer with the crazy bike-tag-along-trailer combo. We lived in town, right on a bike lane, within a mile or two of everywhere I might have to go in town (even rare exceptions weren't more than 4 miles). I could run a nice errand loop and put on 8-12 miles. I felt really good about it, because I needed the workout, I felt better about the kids' being in sunshine and fresh air than carseats, and with the price of gas, I was saving at least half a gallon each time I did that. It was also a great lesson in consolidating errand runs.
Oh--on how it changed my life: it was a contributing factor to my losing about 90 pounds.
Now, I live out of town, but I use the same strategy in running errands, and even ds is excited about biking into town (4 miles) for some fun when the weather warms up.