Just for your info. I've installed a Momentus XT (500Gb) in my MBP 13" (2011) and haven't had any problems with it. It's fast (boot time went from 43 to 19 seconds, mail is started almost before I have lift off my finger from the touchpad etc.).
Fact is that Momentus XT drives with early firmware revisions (SD23 and SD24) are known to have had issues like freezing or not wanting to spin down to save power. My drive (bought couple of weeks ago, manufactured january 2011) had firmware SD24. Before I installed the drive in my MBP, I upgraded to the actual firmware revision SD25. Then restored my MBP from the most recent Time Machine backup. As said, all without any problem. Maybe others in this thread didn't have the latest firmware on their Momentus XT installed?
I love the speed of this drive and certainly don't want to re-install the original drive.
Currently, in the firm I'm working for, we installed the Momentus XT in 5 different laptops as well (cloned the drives). These laptops are all running Windows XP Pro and haven't shown any problems since the drive swap. Boot up times have creatly improved (from about 2 min 45 sec to 1 min 30) and applications start much faster.