I found this excellent post below from BobAtkins.com about using manual focus lenses on Canon EOS bodies via this great article at Photojojo: Better Lenses for Less Money: How To Use Vintage Lenses with Your DSLR
Using Manual Focus Lenses on Canon EOS bodies
A common question is whether older manual focus lenses from other manufacturers can be used with a Canon EOS body. The answer is a qualified “yes” in many cases. Of course you don’t get autofocus, nor do you get any sort of focus confirmation. Also, you don’t get any sort of automatic iris operation. In most SLRs, focusing is done at full aperture, and if you stop the lens down to, say, f11, it remains fully open until just before exposure, then it stops down for the exposure and opens up again. This gives a brighter viewfinder image and makes focusing easier and more accurate. When these lenses are mounted on an EOS body, stop down metering must be used. That means that the lens is first focused at full aperture (for maximum accuracy), then manually stopped down to the shooting aperture before the shot is taken. Some people have trouble accurately focusing using the standard EOS viewfinder screen, since it has no focus aids (like a split image center). While some of the higher end models (like the EOS-1 series, the EOS 3 and the EOS A2), so have additional accessory screens with focus aids (e.g. screen Ec-B has a split image center), the consumer level cameras (Rebel, Elan, digital Rebel, 10D, 20D) do not.
Clearly using a manual focus lens is inconvenient, but sometimes it can be worth it if the equivalent EOS lens is expensive, if the manual focus lens is better than any Canon EF or EF-S series lens (rare, but it happens), if you shoot mostly static subjects or if you don’t use the lens very often.