I use one, but use it less than intentionally I thought. It's
pretty expensive. From time to time to see oe in Ihhbay..
If you have a chance to borrow or rent one before buying do so.
Have a look to the Teleconverter APO 1.4XE as well.
Quality is excellent.
KEH has a 350 CF for $1749. A 250mm + tele-converter would be about the same price but would be more versatile. For my "long lens", I use a 250+2x..image quality is OK..but for nature photography 500mm isn't that long a lens. The CFV, however, will allow really tight crops, with OK results. A 39MP back would be much better but it is out of my price range, so I make do with what I have.