The auto focus is nice
How do you like the auto focus?
My first every Ebay score, the Canon EF-S 17-85mm lens (featuring image stabilization) was a much coveted item around here.  It had come highly recommended and seemed to be a great complement to my beloved 50mm prime and a perfect replacement for the 18-55mm kit lens that shipped with the XTi.Initial, first-night impressions?  Heavenly.Compared to the other lenses that I've put on the business end of the XTi, the 17-85mm is one heavy dude.  I'd wager that it's at least as heavy as the camera and certainly feels as heavy as the 50mm and the 18-55mm put together.  The lens is serious.  It means business.  It traveled back in time from the future to be here with us today.  It's a touch of the photographic high-tech, made accessible to the amateur.With my right hand on the shutter and my left on the lens' barrel, it didn't feel any less heavy but "heavy", in my mind, became "solid".  The auto-focus is nice and quiet and is reasonably fast to lock onto the subject.  The focusing ring is also nice and big -- it cries out to be grasped and twisted.  It was a joy to realize that even with the auto-focus turned on, the lens would not complain if I fine-tuned a given shot with some manual adjustments.  (The AF doesn't totally commandeer the focus, that is to say.)  The zoom is also smooth, the barrel moving quickly and easily without over-shooting the desired focal length.Image stabilization is also a big plus.  I have a feeling that once I get the hang of it, it will save quite a few low-light, indoor, and hand-held shots over the lifetime of this lens.  This isn't to be nave and believe it to be a cure-all.  Certainly a few of my early test shots with the IS feature on prove that you can still wind up with a whole mess of blur.  But others worked out pretty well.After having used it for about two weeks or so now, most of these initial impressions have stayed true.  The IS feature is a good one but isn't something upon which to rely to save every shot.  You still need to have good light and the right angle, etc.; but with the right ISO setting, it can save the scene.  The heavy feeling of the lens is something I've gotten used to pretty quickly; after a couple of sessions, you just expect the camera to weight that much -- no worries.  I've also found that the longer focal lengths are great for portraiture -- I find myself using focal lengths in the 70-85mm range pretty often now and I wonder how I ever lived without them before.That said, I'm beginning to wonder a little about the lens' chromatic aberration.  I've had a few shots that don't seem to "resolve" the same way during post-processing (in Aperture) as similar shots under similar conditions with the same camera body and a different lens (e.g., the 50mm prime).  This is not to say that I have my mind made up and am squaring the blame on the lens -- certainly not!  But I am trying to parse my way through some of these, experimenting with some different conditions, etc. to see where it lands.  I've just fallen into a certain rhythm with my post-processing and I would hate to have to learn a new dance just for the one, new lens.All things considered however, I've been very pleased with most of my results from the 17-85mm thus far....so yeah, worth it.But with respect to "worth it":  Canon lists a retail price of $600 for the EF-S 17-85mm IS lens.  Most places online seem to offer it in the $475-$550 range.  As far as lenses go, this is hardly a costly one.  Considering that this is one of the two lenses "given away" as the kit lens for Canon's 40D body, you wonder a little bit if the mass-production maybe watered it down just a little bit...?  Just the same, this is a great "walk-around" lens:  it is sturdy, has some great built-in features, and has a versatile range of focal lengths.  If you got this lens "free" with your camera body then you are in luck.  If you're looking to pick up one separately...  Well, it's a great lens to have.  And I'm happy with mine.  But see if you can't score one on Ebay or through your local re-seller of high-quality, previously enjoyed lenses.Happy shooting, y'all. 