After much thought I chose this lens as a standard zoom for my 400D. It can produce great images but occasionally doesn't focus correctly, I haven't been able to decide if this is the lens or my technique.
This lens is not weather sealed - on one occasion some grains of sand got underneath the manual focus ring.
I have read a lot about dust problems, but this hasn't been an issue yet, I have taken about 3000 shots, mostly outdoors, since April this year.
On the downside, it is heavy if you are walking around a lot, in these situations I sometimes prefer the 50mm prime. I wish it were a bit longer, but this will be rectified with the addition of a 70-200 at some point.