Every photographer who totes a Canon digital SLR camera needs a medium telephoto lens for that camera. There is no argument about that. It's a given.
The debate comes in when you try to decide which one is right for your needs.
Canon EF 50mm
First things first. We are considering the Canon lens 70-300mm, we are speaking of the f/4-5.6 IS lens, not the f/4.5-5.6 IS DO lens. The later lens costs more than twice as much, but it does not give you that much more value for your money.
So lets take a quick look at the benefits of this excellent piece of equipment. Almost unanimously, buyers agree that they are pleasantly surprised at the image quality. The reason they are surprised is that the lens does not "feel" as substantial as one of the better built "L" lenses.
Another plus is the IS (image stabilization) which will allow you to hand hold your camera for even low light shots. Normally there is a 1/focal length equation that works with a non-IS lens, but with the 70-300mm lens you can add a few stops to that equation.
In addition to the sharp images and IS, you have fast auto-focus. It has a hypersonic motor that allows almost instant focus.
Finally, you will be getting an additional 100mm of focal length. A total of 300mm on the long end is quite impressive, indeed. If you are using one of the crop sensor cameras like a Rebel, essentially that equates to 420mm compared to a full sensor camera body. You can do a lot with that kind of reach.
On the down side, as already mentioned, the build quality is not the same as an L lens. As with almost all zoom lenses, there is some barrel distortion at both ends of the focal range.
Another point that some users find fault with is that the outside of the lens moves during focus. If you are holding the focus ring, the lens can't operate, so you will need to adjust your shooting technique to compensate for that.
Not much in the way of criticism at all. Read the buyer reviews at the online photo stores and at FredMiranda.com and you will see that folks who lay down hard cash for the Canon 70-300mm lens are mostly quite satisfied... to the tune of about a 9.0 rating (out of 10 possible).
On the other side, the Canon lens 70-200mm that compares in price is the f/4 L without IS. As far as performance, it is the f/4 L IS, but the IS makes the price just about double.
Now, there is nothing wrong with either of these two models. After all, they are both L lenses, and they are considered the very best that Canon has to offer.
They are built to last with the very best materials know to photographic engineering.
Images are tack sharp.
Auto-focus is speedy, making them great for action events. Also consider that there is no exterior movement during the auto-focus.
Another benefit is that they have a steady f/4 aperture across all focal lengths.
If your decision leads you to one of the Canon lens 70-200mm models, you will not be disappointed.
Canon Lens 70-300mm Vs 70-200mm - Which One For You