I got a MX510 and a MX518.
The MX510 is 21 years old and got a lot of repairs over the years, the MX518 is around 7 years old (It's the re-vamped "Legend" edition), and still going strong.
Repairs are:
MX510: replaced scroll wheel, replaced pads, replaced top and bottom shell in the.
MX518: replaced pads, replaced switches (for Huano Silent White Dot.
If I ever need a replacement, I'd like to have something almost identical. The problem is, Logitec doesn't make them anymore, and I don't know which one is the closest in shape, etc.