canon

I shot a roll of a film with a newly purchased 75-300 IS and returned it shortly afterwards. I was satisfied with the image quality (although it could be sharper at the 300mm end), however I found the performance of the lens to be unacceptable. The focusing speed is WAY too slow. For those of you who don't already know, there are TWO kinds of USM motors. One of them is very fast and allows focus override, one of them is very slow and does not allow for focus override. This lens has the latter. This lens flaunts the fact that it has a USM motor, but believe me, in terms of focusing speed, you will not be able to detect any difference between this lens and the non-USM 75-300 lens. The fact that the lens is a slow, dark f/5.6 at 300mm doesn't help things either. The image stabilizer does work well, but the focusing motor is just too slow. This is the first lens Canon made with IS and therefore contains the older USM motor (as far as I know, none of the other IS lenses contain this type of motor). Canon should really update this lens and put a better focusing motor in it. If they were to do that, this would be a KILLER lens.

During the event I shot using the 75-300 IS USM, I lost many good shots because the lens kept hunting instead of locking onto my subject. At one point I actually ended up throwing it into manual focus because it was just easier that way. Unfortunately, if you want a long telephoto zoom that focuses quickly, you have to throw out a lot more money and buy something like the 100-400 IS USM (which contains the GOOD USM motor) or the 70-200 USM (also containing the good focusing motor).

I'll close by saying that this lens will *probably* work well for you if you know you'll always be shooting places where there is VERY sufficient light (aka outside). Otherwise, I'd stay away from this lens.

I use this lens with my Digital Rebel 6.3 megapixel camera. I'm one of those shaky photographers, and I knew from past film camera that anything beyond 150mm I would not be able to keep steady enough. I also wanted to have the ability to take photos in low light situations beyond 100mm.
This lense is basically the most affordable that Canon offers with this range flexibility (100-300mm) and with autofocus and image stabilization. Canon makes a wide array of higher end lenses, and some will offer faster image stabilization or clearer picture quality, but for most photographers, you'll find this lense works very well.
If you use the image stablization a lot (like for every picture) you may want to carry a spare battery too, as the IS feature does cause some minor battery drain depending on how much you use it. I found with my Rebel that adding the battery grip with both batteries installed not only lets me take pictures (tons of pictures!) but it also further helps balance the weight of the camera with this lense attached.

Image quality is so-so, and the USM is not ring USM, so you can't adjust the focus manually without turning off the AF. The image quality is not terrible, and is actually quite acceptable.

If you don't need IS, I would recommend that you get the EF 100-300 f/4.5-5.6 USM instead, which is about $150 cheaper, has ring USM, and gives you noticeably better images to boot. But if you need image stabilization in this price range, this is the lens to get, as IS is not available on the 100-300.

This would also make a good lens for the 10D/Digital Rebel/300D, since the 1.6x cropping eliminates the various flaws and distortions at the image edge that you'll see at the longer focal lengths & wider aperatures when shooting with this lens on film.