One important detail you missed... what size sensor? FF or 1.6 / 1.3 crop...
If you're on a 1.6 camera you don't even want to think about how soft the 28-135 is at the edges of a full frame sensor or film...
I've never used the 28-135, but I know my 24-70 is tack sharp right across the frame on film...
Same old story, you gets what you pays for. :-).