I've had both the 17-55 F/2.8 IS and the 24-105 F/4L with my 30D and I like the 17-55 best, by far. Here is why I choose my 17-55 over everything else. The 17-55 is much sharper and faster than the 24-105 F/4L that I had. The F/4 was too slow (for tank shots) unless I bumped the ISO to 1600. It is also much smaller than the 24-70 AND has IS with "L" sharpness. The 17-55 will only work with EF-S mounts, meaning it's only compatible with 20/30/40D, XT/XTi, and the digital Rebel. The EF-s mount won't be going anywhere anytime soon so there will always be a market for this lens if I ever want to sell it. Canon lenses always seem to hold their value so I'm not worried. And I don't worry about the dust either. Mine has a few tiny specs but nothing that will ever degrade the picture quality. I've taken tons of tank pics with my 17-55....just do it, you won't regret it
