Hypothetically speaking, if I were thinking of replacing my 28-80mm lens on my Canon with something a bit more up-market, what would be your arguments for and against:
(1) Canon EF 24-70 f2.8L
(2) Canon EF 24-105mm f4L IS
(3) Canon EF-S 17-55mm f2.8 IS
(1) is the best match in terms of range for what I am replacing, and it's also L-series (which is a major, major plus point) -- but it doesn't have IS, and having experienced it on my new long lens, I rather like it! (2) is slightly longer range and _does_ have IS, but is only f4. (3) isn't an L-series, and is also restricted to use on smaller frame sensors -- no problem at the moment, but if I ever win the lottery and upgrade to a 5D or a 1D (or their successors), I won't be able to use that lens with the new body. (One might argue that if I won the lottery I wouldn't care. ;))
They're all around the same sort of price level; (3) is slightly cheaper than the other two but there isn't a whole lot in it.
Before anyone suggests it, I'm not _especially_ interested in non-Canon lenses -- while I'm sure they've improved a lot over recent years, my experiences with Sigma and other similar lenses have not been all that great.
And before anyone suggests "switch to Nikon": er, no. ;)