I have the 15-85, and like it. However sometimes I wish I had a 2.8 or faster lens in that wider ranges. I just can't pull the trigger on a 17-55, and can't afford the 24-70 2.8 II. I highly recommend the 15-85, but it doesn't do 2.8. If you need speed, you need speed.
We would all like a fast 15-85. Even a constant f/4 would more than satisfy most of us. An f/2.8, even if possible, would be very big, very, very heavy, and very, very, very expensive.