It comes setup as RAID1 Mirrored out of the box, so you don't need to to anything. The mirroring magic is done by the chipset in the drive enclosure as it presented to the outside world as a single regular looking volume, that you can format how you wish.
It should be a case of plugging it in, creating your partitions and formatting.
The only downside, is you can't seed the RAID status from Linux (I haven't found a way at least). It only comes with a Windows utility to do this. Contacting Western Digital about Linux support is about as useful as Anne Frankes drumkit
That said it works well, and does the job, the drives are green, low power and very quiet. However the ability to monitor the status from Linux is a little worrying, as I simply won't know when one of the mirrored drives needs replacing. It's not a short term problem, but it will be a problem in the longer term.
As for my Sheeva Project, it's now also handling all my mail, using Postfix and Dovecot. All my mail arrives on Sheeva, and is stored in MailDir format on the mirrored volume, and it's served up across my several PC's via IMAP. It also hosts roundcube webmail for external access to my mail.