The AI and pre-AI models are optically different.
The pre-AI is supposed to be good.
The AI/AIS is simply excellent: if you don't need 2.8, it nearly equals the 180 ED at similar apertures.
It is light & compact (for a 200), has a built in shade, good ergonomics, 52mm filter and only lags behind a couple of similar FL'd lenses which usually bear the name "Zeiss" or "Leica"...
The only fly in the ointment is that it is effectively more of a f/4.5 than an f/4.0 (which also explains its compactness).
Highly recommended unless you want to shell out for an Apo-Telyt or the like.