I have a 17-55 and will get a 15-85, probably in January or February. I used to have a 17-85, but it was stolen (along with a 70-200 f-2.8L IS, 100-400, Sigma 10-20, 100 f2.8 macro, 1.4 extender, & a camcorder, plus two Think Tank bags & other stuff). I found both the 17-55 & 17-85 useful, but for different purposes. (I realize that the 17-85 is NOT the same quality as the 17-55 or 15-85!) The 17-85 was a great "walking around" lens. For nature shots, it complemented the 100-400 (& sometimes the 10-22). I'll use the 15-85 the same way, though the 100-400 has been replaced by a Sigma 120-400.


I also photograph at horse shows. For some outdoor shows, the 17-85 worked well, along with a 70-200 f/4L IS. (Yes, I had both 70-200's: I got the f/4 first, then added the f/2.8.) Those don't work for indoor shows, though. At first, I used prime lenses: 35 f2, 50 f1.8 (later f1.4) & 85 f1.8. That was OK, especially if I could concentrate on one spot--a particular jump, a barrel in barrel racing, etc. However, a zoom was nice, as the distance can vary a lot. With f2.8, I can barely get a satisfactory shutter speed (1/250, min--1/400 is better) @ ISO 1600 on my 30D. (I can go to ISO 3200, but the noise is much worse.) I still use prime lenses for high-speed events, but the f2.8 zoom is useful for everything else. I just bought a used 50D on eBay ($706 including the BG-E2N grip!) and may be able to use ISO 3200 more of the time, making the f2.8 zooms more useful.