Just to add my configuration.

I have a NAS running unRAID. It runs Crashplan and backs up to their online service. This is where the important stuff is (photos, MP3s, files etc).

My laptop, MythTV machine, work laptop etc backup to the unRAID NAS locally. Some parts also back up directly to the online service.

Crashplan can't backup its local backup store to anywhere else. Not really any point in that. Just backup directly.

Something I've found quite useful is the ability to restore through the webpage. Once or twice whilst away for work on a corporate LAN I've needed a file. I can't VPN out or anything so I just restore it via the webpage to the machine I'm on.

I did also find an issue when the linux NAS didn't have the right codepage causing Crashplan to skip a number of files e.g. "José González". Not really Crashplan's fault though I only found it by doing an audit of number of real files vs number it said it would restore.

Oh and another minor niggle. The backup page counts number of files + number of folders as "number of files"