Delivers on what it has advertized

Like so many people, I bought this to replace my kit lens. In my case that's the 18-55mm IS associated with the XSI. The main issue I had with that lens was the variable (and small) aperature.  I wanted constant 2.8 without sacrificing zoom or IS. That's what you get here. 

The kit lens is also cheaply made in other ways, but the disposability factor of it is actually kind of a plus. Anyway about the 17-55 that we are discussing here:

1. Optical quality. So far, it seems unambiguously better than the 18-55. No surprises there. I don't see any issues with aberrations, though I seldom take photos fully wide open and that's where people tend to see them.  What can I say? My pictures do look a bit sharper.  I think my kit lens back focused a hair, which this lens does not.  The colors look really nice, but it's hard to tell whether that's because the increased aperature allows me to use a lower ISO level or there's something different in the glass.  In any case, the color difference isn't real huge, but everything I can see different in the pictures from this lens is better than I saw in my kit lens (which I thought wasn't that bad).

2. Performance. It focuses quickly and quietly and it has full time manual, so you can tweak the focus after it has autofocused. It's really nice that the focus ring doesn't twist when it does its thing. You can keep your hand on it without worrying about interfering with the focusing. It's an excellent design. I guess that's pretty much true of all USM lenses, but it's really a nice change after the kit lens.  Also the front lens element doesn't twist when it focuses or zooms.  This mostly matters with polarizing filters, but it's just all around nice.  The zoom is fine.  It puts up a bit more resistance than some lenses, but not enough that it's difficult by any means.  In fact, the little resistence makes it easier to get the zoom right where you want it.

3. Weight. It's heavy. There's a huge difference between this and the kit lens. If I were to go on a long walk with my camera, I'd take the kit lens for weight reasons. Besides, outdoors the advantages of this lens are much less. Also this is an expensive lens so I'd never take it somewhere where it might get hurt.  Weight is just one of those things associated with quality lenses, especially zoom lenses with big aperatures.  This doesn't weigh as much as, for example, the 24-70L that I was using (borrowed) before getting this.  I think the lens is as light as you'd want it--it already uses industrial plastic instead of metal in the body.  Any lighter and you'd have to use lose glass somehow. 

4. Design. The front lens is really close to the front, so watch out for fingerprints. I use a lens hood and it keeps stuff away from the glass. It also cuts down on the ghosting, which apparently this lens is prone to although I haven't experienced it.  It's easy to imagine how it would ghost with the glass that close to the front. Like I mentioned, I really just use this lens indoors. The construction has been critisized, but it feels pretty solid to me.  Very solid, actually. It's not metal, but it's heavy, hard plastic.  It doesn't feel flimzy or delicate. I have used L lenses before and unless you are coming under machinegun fire I see no need for the metal construction. People have complained of dust inside the lens. I have been using mine frequently for a month or so (taken a few thousand pictures) and I don't have any dust in there at all. That's not much time, so we'll see. The zoom grip is very large. It practically begs you to use the zoom instead of walking back and forth. And the constant F2.8 helps encourage that. Why physically move when you can zoom without losing aperature?

5. Aperature. I think 2.8 is a great aperature for indoors. It's not so wide that the field of view is so narrow that people's noses are out of focus if their eyes are in. It's wide enough that you get a pleasant bokeh and take pictures in pretty low light. It's an excellent width for indoor shots of your kids and stuff. That's what I use it for. When I want crazy bokeh I use my F1.8 prime, which works well for that. That doesn't happen much, though. The difference between F2.8 and what you'd be using in the kit lens is very noticeable. Totally a good thing.

6. Accessories. I don't use any filters, but this lens really deserves a hood. I can't abide the prices canon charges for their hood here, but on ebay you can get a knockoff shipped from Hong Kong for like 1/6 the price. I did, and it's a really sturdy, well-fitting, excellent hood.  Heavy plastic, excellent fit, looks good.  I see no reason to pay for Canon's lens.  It's not diamond studded or anything.  By the way the lens hood goes on the end of the barrel, so when you zoom in and out it moves along with the lens. On a lot of lenses it attaches to the body so the moving element of the lens moves inside the hood.  Just be aware that this lens doesn't have that feature.  When the lens is fully extended and there's a lens hood on it, it's pretty long.

7. Image stabilization. I'm in the camp that thinks IS is really important. Shaky hands and all that.  I have totally been able to tell a difference on my kit lens and IS is really a good thing. The IS on this doesn't make any noise and it doesn't noticeably drain the battery, so I have it on almost all the time. I haven't really tested it thoroughly to see how well it works, but I assume it works about as well as other lens based IS systems. If so, it works well.

8. An important addition.  I actually can tell a difference in the quality of my pictures using this lens, but honestly it's subtle, not magical.  If you want something that will magically make your photos way better, get the Canon 430ex II or 580ex II external flash and bounce it, along with a DIY or Fong diffuser. That makes a huge difference.  I mean, from nice snapshot to professional image. The external flash is kind of necessary with this lens because the lens sticks out farther than the kit lens, so if you use the onboard flash you will get a shadow in the picture from it.  If you have a lens hood on it, the effect is very much magnified.  Forget the onboard flash if you use this lens, basically. Those lens shadows are not a problem with the bounced external flash. (Remember, this observation is made using the XSI.  If you are using a higher end camera your onboard flash may be different).  Anyway if I had to give up this lens or my flash, I'd give up the lens. But having the lens, I pretty much feel a need for the flash.

Summary. So far this lens has lived up to my expectations. The USM is nicer than I expected and everything else is pretty much what people say. It's a great lens. Overpriced, yes, probably, but it's a real winner. As long as you remember it's just a lens and it can't change the quality of your composition and lighting, you won't be disappointed. I wasn't.