I've had an Xtracycle extension on my older Bridgestone mountain bike for several years now. We use it for grocery shopping, mostly. It is a utility cycling thing, will carry six grocery bags of stuff. It is pretty common to see them carrying kids as well. I have ridden it to work sometimes, and taken in on weekend rides a couple times.

The ride is nice, the long wheelbase makes it stable and relaxed. I have Northroad bars and a Brooks saddle on mine, it is set up for a pretty upright riding position. With 100 lbs of groceries onboard, it is a little top heavy but still okay. A mountain bike's low gearing is essential.