I've shot with the 21mm f/3.5 for a long time. First on the 5D, and now 5D Mark II. Great lens. Recently I rented the Zeiss 21mm f/2.8 ZE to answer this very same question. The Zeiss is sharper at the edges. The Zeiss has a different look. I will be buying the Zeiss soon. That being said, the Olympus holds it's own in the center and middle of the frame. The difference is smaller than I'd imagined. In the outer zones and edges, the Zeiss is clearly superior.
I ended up getting an OM 21 3.5 in EX+ condition from KEH for $440.
One day i might end up getting the zeiss but for travel i think this is a much more reasonable option now.
The zeiss i'm looking at obtaining now is the 100 macro (and selling my canon 100 2.8).
