I have come to really like my setup of a slower, general purpose zoom supplemented with light, fast prime lenses. When I shot a crop body only, I had the 17-55 f/2.8 and it was a stellar lens. However, after adding a 28mm f/1.8, I found I was using the 17-55 mostly outdoors at smaller apertures for landscape shots -- I rarely used the lens at less than f/5.6. The 15-85mm would have been a better lens for this, as it had more range. Since I already owned the 17-55, though, I never pursued the 15-85.
After the above experience, when I went full frame, I selected the 24-105L instead of the 24-70L for its greater range, lighter weight and lower cost. If I need something faster, I will take along one or two fast primes instead. The combination works really well for me.