To be honest, if you're thinking of using the lenses on a EF camera, almost ANY other type of lens mount is better than the FD system. It's almost as if canon didn't want people to be able to re-use their expensive glass on the new system so deliberately designed the EF to be as incompatible as possible, forcing people to buy a whole new set of lenses.
FD to EF adaptors come in 2 flavours - cheap ones with no glass in them, that provide a mechanical fit for the lens and that's all. With these the lens stays on the camera, but won't focus to infinity as the registration distance of the 2 lens systems are so different. For macro lenses this isn't a problem, and obviously, there's no degradation to the image quality as theres no additional lens in the way. The other type of adaptor comes with a lens fitted, which allows the lens to focus to infinity. They're more expensive (much more expensive if you get one with a decent lens in it) and work like a kind of short teleconverter. The one I have is pretty much like a 1.2x converter - making a 50mm f1.8 lens more like a 60mm f2.2, a 28mm f2.8 an effective 34mm f3.5 (you also lose something like 2/3 of a stop of effective aperture
) It's okay if you've got something expensive you want to re-use - say a FD 85mm f1.2 or maybe a fast long lens such as the 300mm f/2.8L, but otherwise :thumbsdown: Mine pretty much was used for testing the lenses I bought from ebay on the digital camera, and since has resided in the "handy to have but not needed every day" bag.
For manual focus lenses that WILL adapt to EF mounts, M42 is probably the way to go, though I have had some success with Contax/Yashica bayonet mounts as well (though the Contax lenses out there are probably a bit steep in price just to experiment with! Nice though
)