I chose the hard disk compared to its price, the guarantee of quality of the brand Western Digital (for information, it is the most reliable hard drives of the consumer market, very few manufacturer returns), and many good criticism.
My use of this external hard drive is mainly the backup: I use Time Machine to backup every 30/40 minutes my entire disk. With my previous Iomega (yet also equipped with a Western Digital Caviar), I regularly heard the disc starting, make backups, and set permanently before. The first thing that I enjoyed with this Western Digital is the silence with which it is done. It's simple: I have to strain to actually verify that the disk works, happiness for me that likes quiet machines. A definite plus.
The capacity of 1.5TB is very very comfortable, although curiously, the greater the space, the more we find stuff to store it, and not necessarily of great use! But being an amateur photographer, I prefer not to skimp on storage, the size of photos being always growing ...
Regarding the absence of on / off button, the disc still connected to my computer, and must always be available to accommodate regular backups, the famous button does not miss, even if it is true, it can be a problem s it is continuously disconnected / reconnected. I rarely get to test the transfer speed, all operations on this disc are made for me and in the background, that said, I have not seen significant difference with other drives USB.
Note that it is recommended to format the drive for use on Mac.