I already purchased numerous flight manuals from eflightmanuals.
First of all, they are very touchy about their "copyright" (which is today apparently defined as scanning in the work of others?), and install some kind of copy-protection on their CDs. When they detect copies of "their" manuals being passed around the web, they get angry, file cease-and-desist orders, and increase the copy protection.
And all over nothing, because second of all, I don't personally find their manuals very educational anyway. This is because (a) the manuals are "Dash-1", that contain no avionics or weapons information - only how to fly the plane, as if it were a commuter aircraft, and emergency procedures for the pilot. The good stuff is all in Dash-34 manuals that are often not secret, but "official use only" - i.e. even in the rare case you can find someone to sell you a copy, they'll want to see proof you are an American citizen.
and (b) because the manuals they have for sale, at least for modern aircraft, all seem to be quite old and outdated. I have the Harrier, F-18, F-15, etc. and didn't find very much in them to quench my thirst.
In general, I've felt far better educated by the free stuff on the web.
My 2c...