At 105mm the lens is better than at 210m wide open.
I believe that this AF zoom lens is optically unchanged from the manual focus version (both are "ULD", and both have the same optical cutaway diagram).
I have the manual focus version and tested it on a Canon 5DII, wide open, focused with live-view, imaging a rich starfield with a star-tracking mount. So basically with thousands of point sources, any aberrations anywhere in the field would stand out very clearly. I tested it at 105mm, 120mm, 140mm, 170mm, and 210mm. These are at roughly equal spacing, in terms of focal length multiples.
What I found was that 120mm is the sweet spot. At 105mm, there seemed to be some very mild positive coma. The 120mm setting showed no position-dependent aberrations at all, within the 35mm full frame. Slight negative coma returned at 140mm and worsened through 170mm and 210mm. I bet most people could use this lens wide-open between 105mm -150mm without noticing anything untoward; I only saw the coma because stars are such demanding subjects. And these were all wide open, f/4.5, so coma will reduce as you stop down.
Central sharpness and field flatness were excellent at all focal lengths. Chromatic aberration was slight - not quite at the APO-like levels of the 120/4 A macro, but close. Vignetting was much lower than with the 120/4 A macro (but that lens is not really designed for infinity use). Overall, the zoom is an impressive performer. Not bad for a $120 lens!
Ray
Sagittarius, flat-fielded stack of images with the 105-210 MF zoom set to 120/4.5