Like the Canon TS-E 90mm f/2.8L, the TS-E 135mm f/4L delivers really impressive image quality.
Note that results with extenders are included, showing that this lens can also be used with those. The with-extender results are nice, indicating that extenders add significantly to the versatility of this lens.
Also note that shifted results are included. To accommodate the tilt and shift movements, the lens must produce an image circle larger than the designed-for sensor format requires. As the lens is shifted (and/or tilted), the periphery of this larger image circle comes into use. Thus, the shifted results have meaning and, in this case, the shifted-"12mm" results show a very slight degradation in the corner.
To see the level of image quality being produced here, check out a comparison against one of the sharpest lenses available: Canon TS-E 135mm f/4L vs EF 300mm f/2.8L IS II Lens. Create your own comparisons.