I beg to differ. If the lens is a true 1:1 Kino Precision lens, it's an excellent lens. Kiron (Kino's private brand) lenses were also marketed under Vivitar labels. They were used also as Lester Dine lenses (of the dental photography specialty) in the 105mm focal length.
In 50mm, a true macro lens is highly versatile, from slow and steady macro work to landscapes and portraits. If anything, the flat focus plane can be used with considerable precision. If you are taking a picture of a pit viper, I'd suggest the longer focal length, but for much work, 50mm is fine. I've found that I use it more that the 105mm simply because it's easier (snaller, lighter) to have on the camera.