I don't use it but it does offer some interesting features.
The $60 (single license) FCP Versioner saves projects as XML files* (which FCP already can do, so I'm not sure of the benefits there) but it also provides a change log between saved versions of a project (which can be handy if you're not already familiar with XML, diff and the terminal) and allows for greater flexibility of number of saves per some length of time. I like autosaving every 15 mins so after an 8 hour day you have 32 saves with only 15mins of work between them. For longer projects or when they're closer to completion I usually change that to about one every hour.
There is a 15 day trial here.
*FCP Versioner's Autosave requires FCP7.