I've heard it before from some staffers' own mouths - E-MAIL is the LOWEST on the "pay attention" scale for these people for communication from outside the office. Hard copy (i.e. letters and faxes) AND phone calls get heeded. E-mail, especially from non-constituents either gets ignored or gets deleted straight into the "trash" box.
Your contacts are absolutely right. Although I have never worked in Congress, I did serve as a staff attorney in the New York State Assembly and the New York State Senate for many years. E-mail is rarely read and even less rarely does it receive a response. Form letters are pretty much a waste of time and postage, except they do provide an endless supply of scrap paper. Faxes will get someone's attention as long as you're not simply faxing a form letter. Telephone calls will generally get the attention of a staffer as long as the caller is sincere and not reading from a script. In my experience, the best way to get the attention of an elected official is to write a personal letter starting or ending with the words, "I enjoyed meeting you at your campaign fundraiser on..." And rather than sending thousands of form letters in support or against a particular cause, you are much better sending a single letter from the leader or organizer of a group of thousands. For example, a typical attention getting letter might start with the words, "I am the president and founder of Hillbillysucks, Inc., a not for profit corporation that now has more than 2500 polically active members who live within your district. I write this letter to protest..."