My sister used to have one (although why she bought it, I'll never know, she's always been lean as a string bean and whatever she wanted to do with her body, good old jogging would have done it for her for less money). I don't know if it's efficient or not, but I tried it, and I honestly didn't like it; all those vibrations left me feeling weird, like I was doing something "unnatural" (I never have that feeling with running or lifting weights). And it was terribly noisy and make the floors in the whole flat vibrate as well. ^^;
So, uhm, maybe trying it before buying would be better, if only to see if it 'fits' you?
The last clear definite function of men muscles aching to work, minds aching to create beyond the single need this is man.
John Steinbeck, The Grapes of Wrath
Color Me Fit