There is, in my opinion, a serious usability problem with encrypted drives conforming to the Trusted Computing Group Opal standard (i.e. the Opal Security Subsystem Class (SSC)). If an Opal-compatible drive thinks power has gone away as part of moving the S3 standby power state, the drive deauthenticates or locks. If an Opal drive is locked, only the shadow MBR and data log areas are reachable on the drive. As a result, the machine cannot see the file system which means it cannot boot out of the S3 state or hibernate. Microsoft Windows will typically bugcheck in this case.
At least one vendor's workaround for this is to prevent machines from going into standby when something happens like a laptop lid gets closed. I believe people normally expect their laptops to go to sleep or hibernate when the lid is closed - not stay powered on. This to me is a showstopping defect for adopting the Opal standard in its current form. The TCG is aware of this problem, and my understanding is that they are working on it.
I look forward to adopting robust standards-based encrypted hard drive solutions later and I hope that standard will come out of the Trusted Computing Group. However, for now I would recommend avoiding Opal compliance. Instead I recommend either using a proprietary encrypted drive implementation like the Seagate Momentus FDE, or stick with mature software-based whole disk encryption solutions.
UPDATE 06 August 2010 - Hibernate mode is not a problem with Opal drives as pointed out by Kris in the comments section. Production Opal drives are due out en masse in Q4 2010. The TCG is working on revising the Opal standard to address the S3 standby problem, but that fix will not be in the Opal drives released in Q4 2010.
Storage Work Group Storage Security Subsystem Class: Opal
email: david @ sharpesecurity.com