I think the problem is probably with that lens/camera combination. I do not have that lens, but I do use a 17-40, a 35-350, and a 70-200, all with not vignetting.

Getting rid of that kind of stuff is a benefit of going with L glass. YGWYPF.

Well...IMHO, yes, but keep in mind that digital technology is a moving target, so unless someone has the same camera and all of the "L" lenses available, it really is impossible to answer your question with absolute assurance.

For me, "confidence" involve always buying a lens from a dealer where I can mount it, inspect it, and use it before purchase. Barring that possibility, be sure you get a good return policy from the dealer if buying mail order.

I have a Canon 1Ds and experience darkened corners when using the Canon EF 28-200mm f/3.5-5.6 USM lens, even when the set focal length is "normal", i.e. 50mm.  I don't experience this problem using the EF 400mm f/5.6 L USM.  In your considered opinion(s) is this a problem with that particular lens, the interaction between that lens and the sensor, or what?  Any one else with a similiar experience?

Many lenses that cover a wide zoom range have light fall off in the corners.  It is a drawback of modestly priced zooms.  It usually only happens at certain focal lengths and disappears if you stop down a couple of stops from maximum aperture.

I'm not surprised that "L" lenses are "the answer", but indeed can I be confident that this is the case?  What are the factors: more careful light dispersion, better assembly, etc. that yield the better performance, or, in their absence, poorer performance?  I chose the 28-200 as a starter lens to give me a single flexible bundle of glass to use while I learned what the 1Ds could do.  Now that I'm past that stage I guess I'll start down the road to "L".  Again, in your opinions, can I do so with confidence?  Thanks.

Almost all lenses have light fall off.  But the question is can you see it.  Better lenses are only 1/3 of a stop or less off, from the center to the corners.  In many cases you don't notice the fall off unless the scene is very evenly lit.  Extreme wide angles are usually harder to correct, so don't be surprised if you see a little in those zooms (17-35mm, 16-35mm, 17-40mm).  But again, it's almost always gone when the lens is stopped down one or two stops.