Vivitar Series 1 28-90mm f2.8-3.5

     Komine made this cult classic,  This is an excellent lens. It's solid as a rock.  All metal and glass except for the rubber focus grip.  It is a one touch zoom.  Focus dampening is firm yet smooth and my copy has no zoom creep.  Focus throw is a short 1/4 turn from near focus to infinity.  Aperture clicks firmly and allows half stop clicks thru the entire range.   
     On my 6D it balances well.  It'll probably be a bit too front heavy on a subframe mirrorless body.   Focusing can be a bit difficult w/out an AF confirmation chip at 28mm.  This is typical for most wide angles.  Focusing at 90mm is effortless.   
     Sharpness is on par or better than primes at all focal lenths and apertures.  That's right, sharper.  The only weak spot is 28mm f2.8.  At 28mm f2.8 you will see heavy corner vignetting and blurry edges.  It is limited to the extreme edges though.  At f4 it tightens up.  Vignetting is gone by f8 at 28mm.  At 90mm the lens is tack sharp from f2.8 up,  corner to corner, with only the extreme edges showing blur.  It is tack sharp at 50mm f2.8.  Contrast is excellent at all settings.  Color rendition is bit cyan, but that is easily corrected in post, or with a good uv or skylight filter.  Bokeh is very good for a zoom lens.   At 28mm it is a bit edgy like most 28mm lenses.  At 90mm it is soft like most telephotos.  Bokeh looks natural for the shot being taken.  I found that f4 is a good setting for natural bokeh at 90mm.  Fringing is well controlled.  It can be seen at 100% crop in high contrast shots at f2.8.  I'll be testing its bokeh fringing performance in a few weeks and I do expect a fair amount of bokeh fringing at 28mm f2.8.   Update:   Bokeh fringing at 28mm is noticeable but much better than its performance at 90mm.   Shoot f5.6 or higher to control color shifts at 90mm.  This is a variable focusing lens.  That means the lens must be refocused every time you change the zoom setting.   For this reason it is important to find a copy without zoom creep.  It is capable of close focusing in a limited fashion.   At 28mm it will focus down to approximately 12 inches.  Performance at close focus is acceptable but it is far from a true macro lens.   Landscapes shot at 28mm f8 will be corner to corner sharp w/ minimal distortion.  This lens seems to be designed for portraits more than anything else.   90mm f2.8 is sharp and I mean sharp.   There is a tiny bit of falloff in the extreme corner but it is barely noticeable.  If you are looking for a crisp, contrasty candid portrait lens, this one will do quite well.