Did you flip the wheel in the cage?
There are two positions for different conditions. Mine came set-up in the opposite position relative to what I required and the directions were not particularly clear. I looked at it a couple of times, thinking it was just fine. The shifting was sloppy and imprecise, as one might expect. I figured it out, flipped it and it works just fine. Shifts just like it's not there.
So count me as "one of those people successfully using Shiftmates."