Thanks for your input, fword.
Weather sealing is certainly something I haven't yet enjoyed.
I suppose what I need to know is whether the 24-105 represents a significant upgrade in image quality compared to the 28-135 which I already own.
Hey, I haven't used the 28-135 before so I cannot comment in detail. The difficulty here is that there's a number of users of the 28-135 out there who are actually satisfied with the performance of this lens. The differences may also be much reduced if you are printing only small sizes or otherwise resizing for display on web. The 24-105 however, even discounting the differences in IQ, is much faster (with a wider aperture) at its long end compared to the 28-135, and the extra 4mm on the wide end could potentially make a difference to you if you are a wide angle photographer. The IS mechanism also is advertised to compensate for 3-stops of camera shake compared to 2-stops in the 28-135. These are significant differences I think, even if the price of the L is more than double that of the 28-135.
