Yes, it is flat on the bottom and has a non-skid pad attached. It is more solid and heavier than it looks in the photos as well. I don’t think it will slide out from under the bike.
If you don’t line up the rollers under the tires properly it can shift a little side to side. I saw a video on it where the dude had it a little diagonal to the tire instead of straight underneath it and of course the tire shifted left and right as he rolled the tire forward and back. I think if you are a little more careful about how you line it up and at least get it fairly straight it could work well.
Alternately, I have seen some folks post comments to the reviews and have mentioned the Harbor Freight Tools Motorcycle Stand. It looks like it could do the job as well and is cheaper than the Wheel Jockey. The Harbor Freight Tools stand only has a 500lb capacity though while the Wheel Jockey claims 700lbs. But it is $30 vs $50 for the Wheel Jockey.
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=98800
I am guessing your friend probably just figures its $15 worth of parts in a $50 package. But thats kind of true of anything you buy retail. If you have a friend who has the gear to weld you up one I am sure that will be a much cheaper option.