[Jim Bryant]

Focus speed

The biggest advantage of Canon lenses over third party ones is not as much the image quality as the Ultrasonic motor (USM). As a generalization, we can state that all Sigma and Tamron lenses are slower to focus than their USM powered counterparts.

How much of a problem is this? It depends. If you are mostly shooting static subject or you are in a controlled environment, you couldn’t care less. On the other hand anything related to sport or wildlife photography requires instant focusing.

In either case, if you are planning to use the lens mostly for cinephotography, it should not be an issue since you will use the manual focus anyway.In this case, just make sure the focusing ring rotation is smooth and fluid.
Internal focusing/zooming

This is one of these things that you don’t notice until it is gone. Basically, it prevent the front of the lens to rotate or moves when you zoom or focus. While this might not seem like a big issue, it is quite important when you are using grad filters. It is a small annoyance when taking pictures, but a big deal when shooting movies. So according to your need you might want to watch this carefully.

Built quality

Canon might charge a premium for its lenses but at least a small part of it is spent toward better quality material. While a plastic enclosure might not have an effect on the quality of the picture, it has a better resistance to shock and scratches. Also, from personal experience I can say that I have never seen a broken metal lens while I have seen countless plastic ones which had a defect of some sort.

While most high end third party lenses have a very good built quality, it is just not as good as the one from Canon. I am thinking here about weather sealing (water and dust). But then again, unless you are planning to drop your lens on the ground, it should not be that much of an issue!
Image quality

Yes, it is true, generally speaking third party lens image quality is lower than the Canon equivalent model. How much lower? It depends. In some case there is not much differences and for others, it is drastic. This is why I created a list of premium third party lens (coming very soon!). While the lenses on this list might not have all the bling and coolness of their Canon counter part, they have identical (or better!) image quality which make them great buys.

Sigma, Tamron, Tokina and several other manufacturers make autofocus lenses for Canon EOS cameras, so why not use one of them? Well, the main reason is that in general the Canon lenses are of higher quality and are more compatible. If there's a problem and you have a Canon lens and a Canon body, then there's no doubt that it's Canon who will fix the problem. If you have a Sigma lens on a Canon body, who has responsibility for them working together? There are also a lot of examples (especially with Sigma lenses) of older lenses not working on newer EOS bodies. Often the lens maker can "rechip" the lens and make it work, but can you depend on that?

Of course 3rd party lenses are often cheaper and some lenses made by 3rd party manufacturers just aren't made by Canon (e.g. Sigma's 15-30 zoom and 50-500 zoom), so if you want one of those lenses, you don't have a lot of choice but to go 3rd party. Many people are quite happy with 3rd party lenses and equally happy to have saved some money by buying them. So I'd recommend sticking with Canon if Canon make the lens you want at a price you can afford. If they don't, then 3rd party lenses are always an option.