I shoot with a 1DMkII, rather long in the tooth (the camera and me, too) but still get very good use of the camera. My 3 most used lenses are the 16-35 L, 28-135 IS, and the 100-400L. Besides having good copies of each, they were calibrated by Canon with my camera at some point, so I have been very happy with their results, even for prints as large as 20x30 and the occasional 24x36.
I have been wishing that the 28-135 went longer or wider, with image stabilization. The Tamron 28-300 3.5-6.3 XR Di VC LD sounds like a walkaround lens that could also replace the 28-135 in the camera bag to cut down on lens changes. This lens is not reviewed on PhotoDo, and I wonder if anybody here has compared it with the 28-135.
I know it is going to be softer at 300 than the 100-400 (mine is pretty sharp) but would it give me quality that is roughly the same as what I get from the 28-135?
I do a wide range of shooting, including street scenes and cityscapes. I also have the EF500 4.0L, so I know what a fine lens can do, but I have been able to make fine art prints using the 28-135, being aware of its limitations.