Thanks a bunch.
Its not too reassuring to hear about the loosening up -- that happened with a poor poor quality Sigma lens of mine, and had some real issues when I was using it. The price increase is pretty drastic when stepping up to the 24-105 L ... maybe I should save my pennies and profit in the long run.
Mine also experienced creep and after a couple of years of use it became annoying, especially with a lens hood and/or a polarizer attached. The focus ring and zoom ring also developped a fair amount of play. I sold it but I would say that it was a nice walk-around lens. The 24-105 is a big step up in term of price, I agree but its contruction is really above the 28-135.
I noticed that my 28-135 produced much warmer colors than my other lenses ... a moot point with digital cameras but with film it was very noticeable.