I have one that I use mostly on Sony A7-series bodies and occasionally on film.
I have owned mine for about 15 years. I certainly didn't pay £600 for it! It's earned its keep - I have made sales of photos with this lens that have earned my investment back several times over.
It's a nice lens, sharp enough with pleasant rendering and no complex distortion. Solidly-constructed with a breech lock mount like all the earlier FDs.
The thing that often gets people excited is the Thorium-doped glass on the concave-front element version, which is mildly radioactive. There's novelty value in that, but also over the 50+ years, due to the radioactivity, the glass takes on a distinct yellow tint.
This makes it fabulous for shooting with black and white film (the main reason I bought mine) as you effectively have a built-in yellow filter without the nuisance of a filter.
For digital use, it can be corrected well enough with a simple white balance adjustment, although I probably wouldn't use it like this for colour-critical work.
If you really don't like yellow glass in your lenses, leave the lens in strong sunlight for a few days and allegedly the UV knocks some subatomic particles back into place and make it clear again.
Some examples, with hundreds more
here if you are interested.
Rose Morris by
Rob Telford, on Flickr
Sandown Pier by
Rob Telford, on Flickr
Flutes by
Rob Telford, on Flickr
Jon Auer by
Rob Telford, on Flickr