I had a chance to try the Zeiss 21mm F4,5 ZM on the Sony NEX5. I used an adapter I purchased off ebay.
I haven't done all that many shots with it yet but this was my first effort. People have mentioned a colour shift on the NEX with these extreme wide angles. I'm not sure if this is a great example to view this, since the early morning light had so many colour's in it.