Here's a quick comparison of the 3 different lenses I have for my Rebel XT, and I also threw in two test pictures made with my old 2.3 MP prosumer Canon PowerShot Pro90 IS.
All test pictures were taken from the same spot. I recently bought two additional lenses for my Rebel XT and I am still learning about them. Thus the comparison.
As it turns out, every camera/lens has its advantages and disadvantages. What surprised me is how good the lens in my prosumer camera is! I have promised to my brother that he can have it, but now I am having second thoughts... As you may know, it has a 10x zoom lens. At 1x zoom it has about the same angle of view as the 18-55mm kit lens of my Rebel XT @ 18mm. At 10x zoom it has about the same angle of view as the 300mm full zoom setting of my 28-300mm zoom lens, plus it seems to have a more consistent sharpness over the entire picture (but the 300mm may have been focusing at a very specific spot - I don't remember now). Unfortunately, it does have some bad purple fringing at that zoom, but the picture I took with it INSIDE with a FLASH was with the semi-automatic setting and the aperture value was f1/3.5 - that's amazing, considering my 28-300mm lens has a maximum aperture of only f/6.3 at that zoom level! (This is probably due to the fact that it also has image stabilization, which allows for bigger apertures.) In short, my prosumer camera's lens has a very good range, fairly good quality and it's pretty fast (i.e. it can work at rather open apertures, letting more light in).
And here is another, somewhat surprising advantage of prosumer/compact cameras. From http://www.daveblackphotography.com/workshop/0502.htm: "This image of Tiger Woods during the 2004 Masters was taken with a Nikon Coolpix 8700. A consumer camera! Its 8 megapixel RAW/NEF file and SILENT SHUTTER capabilities enabled me to capture a high quality image during Tiger's backswing. An impossible image to make with even the top of the line professional cameras due to shutter noise prohibited by the golfers."
As for the lenses of my Rebel XT, each has its own use. The 18-55mm kit lens provides very good wide angle coverage @ 18mm. My 50mm lens has very good picture quality even at the edges (but that's expected, since it's a prime, ie. non-zoom lens) and it's really fast (f/1.8 is pretty fast, that lets in a lot of light in a very short time!). Considering it cost me only $70 (because it has cheap plastic housing), it provides the most bang for the buck of all the lenses I have. I am somewhat disappointed by the shorter focus range of my 28-300mm lens because of its relatively narrow angle of view, but even @ 300mm full zoom it has pretty good picture quality. Even though it has no image stabilization, I've found I can keep it fairly stable. We'll see if that will be true when I am shooting fast moving Formula-1 race cars...
Unfortunately, it's too "nose heavy" - when working with it at 300mm (i.e. when the lens tube is fully extended), my right hand holding the too small grip of the Rebel XT got tired pretty fast. I'll have to find a better grip for this lens that provides not only better support for the lens, but also allows me to move the zoomring fast when action is happening. (Actually, it's quite amazing that with only about 45 degree of zoom ring movement I can go from 28mm to 300mm!).
Anyway, this was not an exhaustive comparison by any means, since it is just a bunch of quick test shots taken at night with a flash inside my house, but at least gives an idea about the different available options.
