I started with Canon's 24-85, a mid-priced lens that is very sharp for the price but not built too well. After 5 years this lens has developed a sticky spot around 50 mm, where it doesn't want to go any further unless it is held upside down (?) .
We just ordered a new 24-85 for my wife's Rebel XT, if it goes as long as the last one we won't be dissappointed. For $300 the image quality is very good.
As for me, I like Canon's 24-105. I haven't noticed any of the problems 0bli0 mentioned but if you read some reviews you will find these are not isolated issues. My first 24-105 went back, it was not as sharp as my 24-85 and for $900 more it darn well should be! My 2nd 24-105 has been to Canon for 'adjustment', it was better than the first one but still not what it should have been.
It appears Canon has trouble making the 24-105, good ones are *really* good but many have troubles. This should not occur on a top of the line $1200 lens! If you want one, be prepared to go through two of them to get there.
Ok, rant mode off, Canon's 24-70 is a really fine lens but too large for my taste, if the 24-105 issues scare you off I would suggest Canon's 24-85.
If you need wider (and IS), Canon's 17-85 is *much* better in reality than the tests suggest. I have carefully looked at images from this lens and yes, it has problems, but less than most comparable 3rd party lenses.
Why Canon and no one else? I have read too many reports of other lenses not focusing properly, needing chip upgrades when a new camera comes out, etc.
ymmv ...