I had a look at second-hand lenses today, Boulevard Beaumarchais ( = photo avenue in Paris).
A 15/3.5 AIS sold for the price of a new 14-24 - out of the question. The lens is very heavy, displays tons of CA, but seems very contrasty. Also, distortion seems relatively low (compared to, say, the 20/2.
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The 24/2 AIS is the exact opposite. Moderate CA, and overall better IQ than I would expect after having read so many negative reviews. Also, it is surprizingly small and light: smaller and lighter than the 28/2. Vignetting is obvious at f/2, but moderate from f/2.8. But:
1) contrast is visibly lower (than the 15/3.5 for instance) - at any aperture
2) the blur circles, away from the center, tend to get an elongated shape, yielding a somewhat nervous bokeh.
3) the distant edges and the corners tend to lack sharpness
Price was 498€; I did not buy it, despite its perfect condition and very nice handling.