I work for a CPA firm. We have a need to transfer relatively large (100mb) files to and from our clients. Our clients include individuals and small companies that do not have access to IT support.
I have looked at yousendit.com, but my concern is that the e-mail notification could be intercepted, and there is no verification mechanism on the site.
My criteria for a product is something very easy to use, inexpensive, and secure.