The main selling point of this drive is that it van be configured as a RAID array. Buffalo amkes differnt sized models. I have the 500GB model (2 250GB disks), they also make 800GB, 1TB, and 1.5TB models. When you use these as a RAID, the capacity is cut in half, but the data is automatically duplicated, so you have a "redundant" copy if one drive fails.  I'll get to the issues with Mac OS X in a minute. Otherwise....

This ia a fast (7200 RPM) and pretty quiet drive. It has a very well built metal enclosure, a\nd there are numbered lights showing you what drive is being accessed.  It has USB and FireWire connectors (not FireWire 800, though). This drive is great value for money/. A downside is that, with two physical drives, it is larger and heavier than most drives of similar capacity.
 
The drive claims to support Mac OS X, but this support is very limited.  The backup and encryption software that os bundled is Windows only.  Even worse, the RAID setup utility provided for Macs works only on Power PC Macs.  While you could connect your DriveStation Duo to an older Mac or a Windows PC to run the utility, this isn't the best solution.  The default FAT32 partitioning can be read by both Macs and PCs, but will not allow for a file larger than 4GB, which can be a problem, partivularly if you use virtual machines, which can be very large files.  SO, if you use a Mac only....

Run Disk Utilitty.  Format each disk as OS Extended (Journaled). This will allow file sizes larger than 4GB.  While still in Disk Utility, click on RAID set and drag the two disks into the box to creat a RAID set from the two drivees.  Now whatever data you copy to this drive will be backed up twice - much more secure but slower.  I use SuperDuper for backing up to this drive, but the backup software shouldn't matter.