I'm a big fan of the 20mm, but one problem I did have, however, was a pronounced degree of purplish vignetting (chromatic distortion) when I took photographs with strong side or back lightning.
I've always read that this can easily be removed in post-processing, but I was never too successful until now. Lightroom 4 makes this incredibly easy. You just drag the little tool over to vignetted area, click and it disappears.
This software innovation has made me much more comfortable about using this lens.