I have one, and they're lots of fun to ride. Almost anyone who can remember to turn it on (i.e., anyone brighter than GWB) can be riding confortably in a few seconds, and feel like an expert after an hour. Whether they're practical or not depends on where you live and where you need to go.
Apart from both appealing to some kinds of gadget freaks (like me), Segways and electric bicycles don't have all that much in common, and some of the least trenchant criticism of the Segway comes from folks who expect them to ride like bicycles. Being on a Segway feels much more like being a pedestrian than it does being on a bike. It's great for shorter distances and (where it's legal) riding on sidewalks.
They were recently legalized in Switzerland, where I live, but this was accomplished by deciding to put them under the same regulatory regime as non-pedelec motorized bicycles, which I think is a very poor choice. Thus, you need a special license for them, some special equipment, and you need to stick to streets. Now that I'm actually violating the law (rather than just testing one of its gaps) by riding my Segway sensibly, I ride it very little.
--Dan