I got my Schwinn '91 MTB converted about a week ago after a 3 week wait for the Xtracycle parts. I could not pick them up in a local shop because nobody sells these in Belgium. But from now on you can, parts and pricing can be obtained here: www.hermesrijwielen.be They did a great job on my bike and even took pictures of the conversion (these will be posted soon).
It is different to ride, at first you don't really notice any difference but it turns differently even without a load. When you do carry a load, eg. groceries, the ride is really steady because the weight is situated low to the ground. Carrying two kids in the back is also not a problem (when they stay still and don't lean in the wrong direction). I took a handlebar from an old kid's bike and attached it under my saddle, so kids riding in the back have something to hold on to.
I just love the fact I don't need a car anymore to do my shopping. This solution is great!