This lens is renown for its excellent optical performance. I have only heard good things about it, and I also did some tests myself which confirm that.
I personally like the little bit narrower angle of view of this 55 mm lens better compared to the standard 50 mm, especially if you also have a 35 mm lens at hand.
This lens was first available for the TL series (with chrome aperture ring), then for the DTL series (shown here) and also as a SX lens for the MSX/DSX series. The lens design (6 elements in 4 groups, symmetrical gaussian type lens) remained unchanged over the years, probably because there wasn´t really anything which could be further improved. This lens is supposedly superior to the 55 mm 1:1.4 version.