I "had" both 70-200 f4 IS and 70-200 f2.8 IS. The f2.8 is literally 2x the weight of the f4, and 80% of the time when I find myself using the 70-200, I would be outside and there is usually enough light for f4.
Also, the 70-200 f4 IS is arguably the SHARPEST zoom lens Canon offers, sharper than the f2.8 IS, at the same focal range and aperture.
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.the-digital-picture.com/R...ns-Review.aspx
f2.8 is faster, no doubt about it, but since you have a crop CMOS body in 40D and not a 1D or 1Ds for sports photography, you dont exactly need f.2.8. (I have a crop body as well in 50D... so. )
The 17-55mm f2.8 IS is also L quality lens without the L, the best available lens at 17 or 55m at f2.8 all the way.
There was a sale on the 70-200 f2.8 and the price difference between the f4 IS and f2.8 IS is about $500 ($1200 vs 1700).
I got my f4 IS only for 1200 shipped from B&H.
So, to answer OP, if you dont have a good low end focal range lens, get the 24-70 f2.8, it is an absolutely amazing lens. I was deciding on this one or the 17-55mm f2.8 when I was deciding on my "everyday" walk around lens..
Obviousy, I chose the 17-55mm... just because I have a crop bodyand 24mm isnt wide enough for "everyday" use..
Eitherway, 70-200 f4 or f2.8 or 24-70mm f2.8 are ALL Great lens. cant go wrong on either of them, although I'd say you need BOTH eventually.
Just so you know, the 24-70mm has no IS and even though it has f2.8 aperture, it is often not fast enough in low lit situations as well and IS feature really HELPS and allow you to go even to say 1/40 without a monopod.
Bottom line: for a crop body, I highly recommend my glass 17-55mm f2.8 IS USM. Unless you're thinking going FF soon, then you should get the 24-70mm. Else, while everything is perfect, the 24mm just is not wide enough and you'll feel it if it is an "everyday" walk around lens for you.