50mm lenses typically use a age old optical formula which is not as good as some of the newer formulas using more elements.
You are paying more $$$ for supurb construction and for the Name. For manual focus, Zeiss lenses are wonderful to use.
Photozone previously tested the lens on a crop, but it has fallen off their list, probably because of the obsolete camera.
Here is what they say. "Some years ago we analyzed the Zeiss Planar T* 50mm f/1.4 on an APS-C DSLR and the results were truly impressive there. However, on a full format DSLR the performance leaves a bit to be desired actually. "
As more and more users switch to FF, the lens will become less popular, so its price may suffer, but thats just a guess.
DXO has also tested the lens on many bodies. You can view the test results and select different bodies to compare here:http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Carl-Zeiss/Carl-Zeiss-Planar-1.4-50mm-ZE-Canon/(camera)/185/(cameraname)/Canon-EOS-450D
Their test data is fine, but their DXO Mark ratings are biased, just as is any rating that tries to give a overall score for a product is biased by the method used to score it.