I never buy the fastest. 9 out of 10, the second largest aperture lens is the best one, with better sharpness, less vignetting, less distortion, less dick-looks (hey, let me put this dick in your face when I make a picture!) especially if you include the much-needed hood on a large aperture lens, a full open quality comparable to the larger one stopped down to F4, and best of all, much cheaper.
I did a very careful comparison between the Nikon 85mm 1.4 AFS, the 85mm 2.0 AIS and the 85mm 2.8 PCE, and the last one delivers a knock-out to the first. It even beats it for portraits, as it has the nicest bokeh of any lens I own. Full open the 2.8 PCE is totally sharp, where the 1.4 gets comparable corner-corner sharpness from 4.0-5.6. It is very logic too: the larger the aperture, the more compromise in the design.
Another problem of that 1.4 is the closest focus distance. It can't even focus a face up close! With the PCE you can capture just the mouth if you like. And because of the closer focussing distance, you get creamier bokeh and more out-of-focus at 2.8 than possible with the 1.4! The 100$ 2.0 AIS on the other hand, is still the best-value 85mm out there. The 1500$ of the AFS 1.4 does not make sense.
Another thing, most of the time, the uber-large aperture ones have such a horrible vignetting, that when the center is for example 1.4F stop, the corner is already more like 2.8. So why not get that 2.8 then? I don't want dark corners.