Andy Westlake, in his DA* 55/1.4 review conclusion:
… that 55mm focal length is offering the angle of view of a classic 85mm lens from the days of film, giving you a slightly longer working distance and more flattering perspective compared to a 50mm prime on APS-C. All in all, the lens is a clear improvement over the smc FA 50mm F1.4, with much superior wide open performance than the older, film-era design (although the gap narrows considerably at smaller apertures).
However …
The SDM focus motor is near-silent, and won't disturb your subjects in the slightest; however it is slower than the screw-drive system of the older 50mm, and while this isn't a big deal if you tend to pre-focus then shoot, you might need to bear it in mind if you use autofocus to track continually and erratically moving subjects – children, for example.
Read more on the DA* 55 f1.4.