We have been using Prevent for email since late 2007. We chose to include that with our original DLP implementation for PCI compliance reasons. Our support team did a good job planning for the redundancy we needed, but we also made a "policy decision" that included agreement to use a "fail open" emergency response if we ever hit a problem that interrupted our critical email flow.
So far, our support team has been able to avoid such a "drastic" measure. They've watched for queues and managed managed the performance very well. ;-)