I've got a pair of heelys. They are fun, but they aren't significantly faster than walking. Also, to get get your legs locked out in able to cruise around on them your hamstrings will get tired. So in terms of saving energy, they aren't really sufficient.
ps. I've never went backwards on them.
pps. Get them. It might not be any better than walking, but it is still fun. I'd wear heelys at my job if I could.