I have adapted MANY lenses for use on my 1Ds Mark II, and before that, my 30D. Yes, modern EF lenses will be easier to focus since they have autofocus, and aperture is fully manual with an adapted lens (You focus and then stop down manually to take the picture).
That said, some truly amazing glass is out there from older mounts at a fraction of the cost of the Canon equivalent, and in many cases are better.
The mounts that are easily adaptable to Canon with a simple adapter ($20 or so on eBay or Fotodiox, or about $80 if you get a focus confirm chip, which will let you manually focus, but have the camera confirm when focus is reached (and provide EXIF data in the file matching the lens):
-Contax/Yashica (Carl Zeiss lenses) - Some of the best lenses around…the 50/1.7 Planar and 85/2.8 Sonnar are relatively inexpensive ($200-$400) and are truly exceptional…best color and contrast of any lenses I’ve used.
- Leica R (not M mount, but the SLR R mount)
- Olympus OM (Check out their 24/2.8 for a pittance, and one of the sharpest 24mm lenses out there…also, the 50/3.5 Macro can be had for about $100 in mint condition and is razor sharp…use that regularly on both my Canon kit and my micro 4/3 kit)
- Pentax Screwmount (M42 screwmount) - Tons of great old lenses…The Takumar 50mm f/1.4 is one of the best all around 50s made, and it’s tiny and built amazingly well.
- Nikon F. Modern Nikon lenses don’t have an aperture ring, and so aren’t easily adaptable without spending big bucks (there’s an adapter that works with some lenses like the 14-24, but the adapter is about $200). A simply mechanical adapter will work for all the AI, AiS series and such lenses…tons of good optics here.
Here’s a good guide to adapters and using manual focus lenses on Canon. Be careful, though…it can start serious gear collection. I’ve had about 30 old lenses for use on my various cameras. Some are junk, but some are real steals and amazing optics. They also often have a very different ‘look’ than Canon lenses, which is often what people are after with these. Zeiss lenses have rich, saturated color and very high microcontrast…makes things pop out amazingly well. Olympus lenses tend to be more muted and natural in their color rendition and have a shallower contrast curve, etc…
FD adapters are available, but have optics in them to get to infinity focus, and act as a teleconverter. Most of them are also quite poor in image quality and would ruin any reason to use the lenses.