I like to back up the entire project every so often, and if the media is stored 'outside' the process is very quick and efficient - I'm only backing up the changed data, not ALL the rushes, and in this way I don't have to pull off drives. If I didn't use SparseImages, FCPX gets confused with having a duplicate of a project on the system and won't play until one of the duplicates is actually physically removed.
So, to do a quick backup, quit FCPX, drag the SparseImage image to another drive, and within seconds it's done. Relaunch FCPX and you're away.
Also, when sharing projects out, handy to have one central Rushes folder that's common to all editors.
But if the SymLink is futzy, and there's a better way of backing up...