I just bought second hand EF 80-200 mm f/2.8L lens, and therefore replaced my EF 70-200 USM f/4L.
Lens it's quite old, perhaps 20 years or so, but build quality is just stunning, like current L series, but for about thousand pounds less. I am not using this type of lenses often, so this is good replacement, at actually no cost to me. Usually the newer the better, but i will now sell my few year old lens and stick to that great one!