As I owned and used the 17-55mm f2.6 as well as the 17-35mm f2.8 and 28-70mm f2.8, I will pen down my personal opinion.
While the 17-55 offers a wider range, hence more flexibility convenience. I find that the other 2 lenses to be sharper (do not be mistaken, the 17-55 is sharp). So why do I still used the 17-55mm? In certain situation the convenience of having a single lens to cover a wide range outweigh the need for very sharp pictures especially when it is not going to be printed in large prints. Of course I am hanging on to the 17-25 and 28-70 so that it can be used when Nikon FF DSLR come.
I shall now close this thread, as it has gone long enough for the starter to make a decision.