I've had the 1 TB version for months now. Never had any of the sleep issues some people have mentioned.
Only slight problem I've ever had with it is it would have problems dismounting. I'd have to quit finder and then eject the disc a second time, which would always work.
This wasn't too much of an issue for me because the MBP the drive is hooked up to is generally used like a desktop these days; i.e., monitor, mouse/keyboard, ext HD, so it wasn't dismounted too often.
However, that problem went away when I installed the latest firmware about a month ago.
I leave it powered on all the time to let Time Machine do it's thing ( I have it set up with 2 partitions, one for TM).
Overall it's a good drive, but, I think next time I'll go back to what I had before, meaning I would get a external FW800 enclosure and be able to use whatever hard drive I want (easy HD upgrades).