1TB WD Mybook here, too. No problems. It will spindown occasionally if you leave a game paused forever, but it spins back up with no problem EVERY time. The biggest issue you should ever see is a stutter of about 2 seconds when it spins back up (which is quite rare unless you often pause and take off for a while).
My drive is split into 3 partitions: 400GB for FAT32, for playing media through MPlayer-CE, my dvd player w/ a front USB port etc, 400GB for WBFS (which is PLENTY.... I've got 60 games installed and have barely dented it), and 200GB NTFS for my files over 4GB.
MPlayer-CE is a little buggy with it (freezes occasionally), but I think that has a lot more to do with MPlayer than the drive.
If you're looking for a smaller drive that doesn't require another outlet (ie: powered by USB), my brother has a 320GB WD Passport that's awesome, too. Best part is it comes in white/silver and perfectly matches the Wii... and there's a forwarder that uses an image of that particular drive, so everything looks nice and professional.