I recently received a Bass Buggie from KC Strings. I'm impressed: It's easy to attach, easy to remove, and moves great in traffic. The fact that you can strap the buggie to your bass without any modifications (i.e., removing your endpin) is great.
I had two gigs yesterday and needed to walk my for a couple of blocks downtown (in both pedestrian and vehicular traffic). I know I could have done it without the buggie (I have before), but it was so much easier with this little contraption. Plus, it looked way cooler than huffing and puffing with my bass on my back. Moreover, given that I usually carry my bass using the shoulder strap or one of the handles on my gig bag, the buggie may actually extend the life of my gig bag as it won't have the stress on the strap or handles from long walks carrying my bass.
Kudos to KC Strings.
The buggy is priced at $110 or so, which seems comparable to the price of wheels (ranging between $75 and $136 depending on what you're looking for). That said, the practicality of the buggie over a bass wheel make it well worth the price.