I upgraded from the kit lens to the 15-85. I was able to use both the 17-55 and the 15-85 at the same time. My skills are limited, but as to my experience I noted the following on the XS body I had at the time.
I could not tell IQ apart at identical lens settings Contrast, and sharpness. both were far superior to the kit lens. I think they both out resolved the XS sensor
both lenses focused very fast, and accurate. I had no USM lenses previous to them so all I can say is that compared to what I had on hand - they were both lightning fast, telling the difference between the two was impossible for me.
I did notice that the 17-55 id flare more than the 15-85. I felt that 15-85 was more forgiving of photographer position if you're a little less experienced.
weight between the two was pretty close at least in a day of "real feel" type testing
outdoors, in the daylight both lenses are excellent. at night both also focus well, but the 17-55 produced nicer photos due to the larger aperture allowing lower iso all factors being equal. but again, when I was taking shots of the cookout. the yard lights and firelight created a few streaks in the image.
at the end of the day I went with the 15-85 for 2 main reasons:
1. for some reason the 15-85 felt like a more useful range for me - it wasn't until much later that I realized that the P&S I had learned on (first camera -ever) had a 35mm eq. zoom range of 28-140. no wonder. my only experience with cameras was a nearly identical range.
2. after looking at the kind of pictures I liked the most and kept the most often - I realized that I was likely for long into the future going to be a daytime outdoor fun-in-the sun photographer. with a little bit of indoor party and odds and ends snapshots mixed in. I had a good flash and know a little bit (well, a really little bit) of how to use it so for me the low light just wasn't as important as the range. 15-85 is an awfully handy range for the occasional family camping/travel weekend. The less stuff to take along, the better.
as I get better and add more challenging situations to my shooting, I will probably realize the added benefits of the larger apertures. Hwever for now I feel that a simple fast prime will take care of most of my current needs