Both Yashica and Zeiss produced a few zooms that were every bit as good as the primes covered by their focal lengths: among these are the ML 28-50, ML 35-70 3.5 & f4, Zeiss 35-70 f3.3, 35-135, 100-300. All superb performers and worth paying a little bit more for the pleasure of the piccies they could deliver. This lens is the finest of them all.
The 45-90 is approximately a 28-60mm in 35mm terms - that is strictly relevant because use of the Contax NAM-1 adapter permits the use of the Contax 645 range of lenses on the Contax N-series 35mm SLRs.
While everyone knows the qualities of the truly stunning 80mm f2 Planar for the C645, if you don't need its special qualities, this zoom will fulfill almost all your needs for general photography. It is my go-to lens when I'm out and about with mo specific photographic objectives in mind. It's incredibly sharp, evenly illuminated and largely distortion-free - but it is quite hefty: 1.14kg. However, it balances nicely with the big Contax and eliminates the need to carry both a 45mm and 80mm lens.
Such is the reputation of the Contax 645 glass that a variety of adapters is available to permit it to be used on modern, digital cameras. I can confirm that the lenses work very well, though slightly slowly, with the Fringer AF Adapter on Fuji's range of full-frame medium format DSLRs.