neuroanatomist: For a crop - t2i - would you recommend as a "walk around lens" with IQ in mind not budget, the 16-35 ?
Actually, I'd recommend the EF-S 17-55mm f/2.8 IS as the best general purpose zoom for an APS-C body. On the same APS-C body, the IQ of the 17-55mm is better than the 17-40L, slightly better than the 16-35L II, plus the 17-55mm has a broader range and IS. With a 7D, the weather-sealing of the 16-35L might be a factor, but not with a T2i.
I'd 2nd that, IMO it's sharper than the 24-70, I was really surprised the first time I played with it. Also it's an effective However I remember having my T2i and being pretty damn impressed with the 16-35mm on APS-C, the bokeh is very circular and looks beautiful cropped.
But in terms of versatility, Neuro is right, the 17-55 is awesome. The only bad thing I've ever heard about the lens is that its somewhat susceptible to dust.