At the moment, I'm using the a vitacon 0.5x 58mm on my Olyc2100, I do get vignetting at the corners and to be picky, I find the edges slightly soft. But it's cheap so can't really expect much.
I had a glance at the kenko, found it almost identical in make to the vitacon, not sure what's the difference in quality exactly.
If you're interested I can always let you try a few shots with the converter and let you decide if you can live with the photo quality.