I don't do a whole lot of telephoto shooting, but it's kind of nice every once in awhile. I was thinking about getting a 300mm 2.8 and mounting on an EOS body. I could get a 300mm 2.8 FD and have it converted for roughly the same amount as a Nikon 300 2.8 ED AI(S)+adapter.
Even if the FD lens convert to EF mount is the same money as the Nikkor, the choice to get the Nikkor is better because it can be used on 2 systems, Canon and Nikon, not to mention the risk of the conversion. And later on if you don't like telephoto shooting, reselling is also easier.
I picked up an SMC Takumar 300 f4 in m42 mount. Haven't used it a great deal, as I don't currently have a body (awaiting 5D MkII), but it was very cheap and it was indeed plenty sharp enough on a 5D. Some say the bokeh on these isn't fantastic, but it looked ok to me (again, I only used it for a few hours the day before I sold my 5D).
It's quite light, a manageable size and has a built in hood and tripod collar. Old rumours have it that the SMC coating is identical to Zeiss' T* coating, though I have no idea how accurate they are. If you don't use 300mm much, it may be a decent option, as it's less than $300.
Another option is Tamron adaptall 300mm 2.8 SP
It is a better build lens than the Nikon 300 2.8 AIS
It is sharper but slightly less contrast than the Nikon.
Nikon 300mm 2.8 AIS vignette at wide open.
The only negative about the old Tamron is CA. More visibile in high contrast light.
