Personally, I use the 17-55 f2.8 IS for my wide-ish to long-ish zoom. It is a very good lens, without a doubt, and I find the 17mm end quite wide enough for me (but I never went much wider than 28mm in my 35mm film days - which do in fact continue).
However, that lens is quite expensive (and quite large); and while constant f2.8 max aperture throughout the zoom range, and image stabilisation, are very nice to have they may not be worth the additional cost to many people.
I have used my father's Sigma 17-70 f2.8-4.5 and can confirm that's also a good lens, and quite a step up from at least the old Canon 18-55 non-IS kit lens (I've not used the new IS version). I suspect this would be a very good compromise on zoom range, price and size for a great many people.
I also have the Canon 28-135 IS lens, which I used extensively in the past but use quite infrequently now. I used to use it as my walk-around zoom lens and found it optically decent enough but not great. Eventually, with an expanded range of lenses to choose from, I found it not wide enough at the wide end and not long enough at the long end, while not being of as good optical quality as some of my other lens choices. I use it from time to time on my EOS 30V as my film zoom option, because it provides a good zoom range for 35mm film, but even there I'm more likely to use my prime lenses (I use film mostly for black and white, BTW).
...Mike