In my case, the Canon EF 75-300mm f/4-5.6 III lens came as part of my T2i / 550d kit lens, along with the typical Canon EF-S 18-55mm f/3.5-5.6 IS.
I've had it for nearly a year now, so I feel I played around with it enough to know most of it's strenghts and limitations.
The Canon EF 75-300mm F/4-5.6 III lens brings a nice zoom range, giving 120 to 480mm coverage on my T2i. That's plenty of zoom for most situations, and I rarely ever wished I had a 1.4x or 2x extenders.
Weighting in at 16.5 oz. or 480g, this lens is in the lightweight class of the telephoto world (though noticeably heavier than a 50mm f/1.8 or even the EF-S 18-55mm IS).
The price was also excellent, with the Canon store selling it at a modest 199.95$.
Unfortunately, that's about all the good things I have to say about this lens.
The auto-focus is very hit-and-miss, and I find myself keeping the lens in manual focus mode most of the time. The focus ring is rather small, which makes precise focusing hard, even for immovable objects such as the moon, and the filter ring moves with your focus, which makes some filters difficult to use with it.
The image quality is absolutely horrible, though after further research, I can finally understand why.
When we shop around for a camera kit, everyone asks for the resolution of the camera. The T2i comes with a very adequate 18mp sensor. What most beginners don't know, though, is that lenses also have resolution. Lens resolution is not measure in mega pixels, but in a rather weird line widths per picture height (LW/PH) which you'll see as MTF charts. How many LW/PH translate to how many mega pixels is still a mystery, but if a lens' MTF results are too low, you can expect the lens not to perform to the standards of a very high mega pixel camera.
Lets go back to our 75-300mm III for a moment, specifically at it's release date. The 75-300mm was released in 1991, whereas the III was released in 1999. To give a good appreciate of the III's image quality, take a look at the speficiations of the Canon EOS D30 which came out in 2000 (not to be confused with the D30 which came out in 2006). The D30 had a 3.14 mega pixel sensor. Even the EOS-1D which came out in 2001 had a 4.15 effective mega pixels.
There are a myriad of alternatives to the 75-300mm III from Canon. The Canon EOS 55-250mm f/4-5.6 IS comes out at about 100$ more, and comes with IS and better optics.
There are also many Canon 70-300mm lenses which supposedly have a better quality, and you can get them with IS or USM.
Then there's the ever popular 70-200mm gambit of lenses. Canon wins the first place of such lenses, but they come at a very steep price. Tamron's 70-200mm f/2.8 wins a special mention for having an even better image quality than the Canon lenses, and for having the cheapest price, but comes with a mostly un-usable autofocus motor. Sigma's 70-200mm features a very adequate HSM motor for fast auto-focusing, but it's image quality isn't as strong as Tamron's and Canon's.
Then there's also a big selection of primes from 200mm to 800mm, each costlier than the other, but with great image quality, and great weight.
I'm still undecided what I'll trade in my Canon EF 75-300mm f/4-5.6 III for, but I know that out of all my lenses, it is the one that limits me the most, yet my 18-55mm f/3.5-5.6 IS is in the same price range, and my 50mm f/1.8 came at half the price. Almost every time I pick up my EF 75-300, I end up disapointed in the results, so I would recommend everyone passing on this lens, unless you really can't afford anything better, and understand that your lens will be limiting the quality that your camera could process.
Morale of the story: The Canon EF 75-300mm F/4-5.6 III lens could be a decent lens if you use it on the 1, 2, or 3 mega pixel cameras it was designed for, but in 2011, I don't think this lens should still be on the shelves.
(I'll update this post in a few to several months when I finally have an upgrade)
Cameras: Canon EOS Rebel T2i, Canon S90
Lenses: Tamron: SP70-300mm F/4-5.6 Di VC USD, Canon: EF-S 18-55mm f/3.5-5.6 IS Lens, 75-300mm f/4.0-5.6 III, and EF 50mm f/1.8 II
Retired camera: Fujifilm Finepix s700