Madwand is right-on with his lens review of the Tamron 26-300 Di. I would read it thoughly and decide if the compromise for weight is worth it to you.
I use my Tamron 28-300 Di lens on my Canon 1DMKII and love it. The Zoom range and weight is why I bought the lens. I have framed some 13x19 printed photos and noticed some chromatic abberation at 300mm, but then again, I wouldn't have got the shot without the Tamron 28-300 Dirange (on a tour tram).
I also have other lenses that are used on special occations ... 17-40L, 28-135IS and 100-400L, but I find myself using the Tamron 28-300 Di as my everyday carry lens.