I've had my 20D for over a year now and have been using my Tamron 24-135 with it the entire time. I bought that lens about 4 years ago (coupled with the Elan 7e) since I was on a budget and was looking for the best "bang for the buck."
While I've been very happy with the Tamron, I was now looking to replace that lens with something better, assuming that a Canon L lens would fit the bill. After reading all the reviews, I bought the 24-105 on a good deal from Dell.
The look and feel of the lens is very nice and it's a pleasure to shoot with, but how about the performance?? After shooting back to back against my Tamron for days, I was VERY surprised to find that I actually preferred the Tamron for outdoor shooting. The Tamron was sharper and clearer across equal focal ranges and apertures. The difference was usually very slight and only noticeable at 100% crop comparisons . . . but my findings were consistent. That being the case, why pay $1100 to replace a lens that it can't beat??
The Canon, of course, has the edge in indoor and low light situations since it has IS, but this alone was not enough for me to keep the lens. I returned it and am delighted to keep my Tamron as my outdoor walk-around lens. I will be using the mone saved to buy an UWA or telephoto zoom to complement the Tamron's 24-135 focal range.
If low light situations are few and far between for you, do youself a favor: buy the Tamron 24-135 and have fun shopping for something else with the extra $800 in your wallet.