As a portable drive, the Mirror is bus-powered, meaning it doesn't require a separate power adapter. Instead, the USB connection will also draw power from the host computer. But this doesn't really matter much, considering you might not want to carry it around anyway, for the reasons mentioned above.
Easy-to-set-up, helpful software
When plugged into a computer right out of the box, the Mirror appears as a small FAT32 partition, just some 200MB, that contains the LaCie Setup Assistant software for both Windows and Mac. You need to run this software before you can use the drive's entire capacity.
The software walks you through a few steps of different options. These include making the Mirror work as two separate partitions, one of the current platform -- using either NTFS file system for Windows or HFS+ for Mac -- and the other for sharing cross-platform using the FAT32 file system. (Note that FAT32 file system can't hold files that are larger than 4GB each.) You can also choose to install its backup and security software.
Both the backup and security applications work well. On a Mac, you can use the drive with Time Machine, however. For data security, the drive supports AES 256-bit software encryption; just make sure you remember the password if you choose to use this feature. Otherwise, there's no way you can access the data.
Note that you can ignore the Mirror's software completely and manually format it using existing tools within Windows or OS X, which took just a few seconds in my trial. Basically, the drive works like any portable drive on the market.
The Mirror didn't blow me away with its performance, but it wasn't slow either. Via USB 3.0, it registered a sustained real-world speed of almost 100MBps for writing and slightly more than 110MBps for reading. These were on par with most hard-drive-based USB 3.0 portable drives on the market.
The drive also worked with USB 2.0 with the speed of around 30MBps for both reading and writing. Overall, it worked well in my tests with no issues whatsoever.
The LaCie Mirror Portable Hard Drive is a bit odd, to say the least. The design, though unique, adds no value to its functionality as a storage device, and its sheer amount of reflection might make you wonder if it was designed by a narcissist. During testing, I found the reflections rather distracting and at times annoying, especially when I left the drive in the sunlight.
What bothered me the most, however, was the fact that you can't hold the Mirror without making it dirty, which defeats its purpose of being a portable drive, something you carry around frequently.
In the end, whether or not you should buy the Mirror depends on if you find unique design worth the extra cash. As far as storage is concerned, note that you can easily get a portable drive with double the Mirror's capacity for less than $100. Nonetheless, once you've gotten your own, you'll find the Mirror a quite good portable drive that works as intended, both as extra storage space or as a backup destination.