Mary Anne, have you had focussing issues with your 24-105? Mine is near perfect (well, perfect enough for me, but I am sure I will get fussier as I improve), but a friend of mine has reported some issues with his. He told me his focus tends to be a bit out for no apparent reason and it is not always in the same direction (not always back focussing or front focussing). The early models of these lenses apparently had a flare issue which could be corrected by Canon service. Shame, because they are quite sharp and have quite low distortion (for the range of zoom) and vignetting is pertty mild.

So far, mine is good (fingers crossed).