Canon have made 5 versions of that lens.
Canon FD 400mm f2.8L manual focus lens for FD system
Canon EF 400mm f2.8L autofocus lens for EOS cameras
Canon EF 400mm f2.8L II autofocus lens for EOS cameras with Fluorite element. This lens is reviewed here
Canon EF 400mm f2.8L IS with image stabilizer
Canon EF 400mm f2.8L II IS latest lens with power focus mode for use when shooting video, IS-3 mode for the 4-stop image stabilizer, improved image quality and flourine coating at front and back glass surface to repel dust and moisture
The first two 400mm f2.8 were not that good optically. This could be due to lack of any Fluorite element. The last three 400mm f2.8 lenses use one Fluorite element and have excellent optical quality. For the last 3 lenses, camera sensor, subject movement, atmospheric haze and photographers skills are more likely to be the limiting factor.
As canon will no support the 1st 3 lenses, i.e. spares etc, they only keep parts for up to 10 years life of the lens, so if you get a problem, repairing the lens, might become a major issue