I use a box at a UPS store, so I can accept UPS and FedEx in addition to mail. They also make it easy for my tenants to drop off the rent in person and just have the person behind the counter put it in my box. I get an email when I get a package or mail in my box, I think it's pretty convient. I never tell my tenants where I live.
Leaving aside privacy issues, which as many have noted, are easily discovered... the separate P.O. box creates a clear division of the company from your personal life - highly desirable both from an LLC and from an IRS perspective.