I owned this lens for three years before swapping it for a Tamron 28-75 f/2.8. I used to praise this lens...
until I got to photograph with the Tamron - the difference (especially in the image quality department) was like night and day. Somebody had switched the lights on! It had opened a whole new world of creative opportunities for my photography.
The Tamron offers superior sharpness, colour rendition, nicer bokeh, smaller amount of vignetting and it allowed me to take pictures at f/2.8, which meant better low light capabilities and small depth of field. It is almost in the same price/weight league - the only area in which the Tamron is worse than this Canon is the noisy AF. The Canon seems better built at first glance, but it started to wobble after 1 year of use (the Tamron wobbles too, but less so than the Canon after years of intensive use and world travel).
The same applies to Tamron's 17-50 F/2.8 for aps-c digitals. It provides WAY superior IQ compared to the Canon 28-105 and it has a more appropiate range for small sensors (300/350/400D, 20/30D).
I have also tried the Canon 24-85 and my conclusion is that it is slightly sharper than the 28-105.
Canon's 28-135 IS is sharper than both the Canon 28-105 and the Canon 24-85, but it is still behind the Tamrons.
The bottom line is:
Both Tamrons mentioned above and Canon's own 24-85 are superior alternatives in the same price range. The tamron's offer L glass image quality of you obtain a good copy for a marginally higher price.
I do not recommend this lens - there are better alternatives out there.