From DP Reviews studio tests:
Distortion is a problem at almost all focal lengths. The 18mm setting displays an extremely high level of barrel distortion (2.8%), which changes rapidly to pincushion by 24mm, and becomes extreme from 35mm to 70mm (peaking at 50mm and 2.3%), before diminishing again towards the telephoto end.
The distortion shown by this lens in our studio tests was so marked that we felt it merited illustration with some real-world shots. Barrel distortion at wideangle isn't unusual on zooms, but this lens is worse than most, and it has a complex 'wave' pattern with recorrection towards the corners, making software correction relatively tricky. It's also rare that we find pincushion distortion so pronounced as to be visually disturbing, but that can be the case between 35mm and 70mm; the good news is that the pattern is simpler, and can be readily corrected in software if desired.