charles, I'll bump this for you.
Research the companies, but generally speaking, your confidential data is probably safer at one of these companies than if you have your own network/db. IF you have need to retrieve/compare and manage the documents, electronic dbs are going to assist versus paper. Plus, you'll likely have the advantage of Amazon/rackspace etc. continuing to improve their hardware, upon which many of the management companies rely.
In my legal practice, we are comfortable with off-site databases for our larger document cases, and that is common in the US legal field, despite extremely high confidentiality concerns. So too, some of my physician friends have used remote record services--and I think they are happier than DW is with her group's service, which is hosted on their own server.
Perhaps this bump will get someone more knowledgeable to chime in....