To obtain a login, you'll need a customer service number. then the rest of the registration process is pretty simple.
The documentation it contains is a slew of white papers, TARs (Technical Assistance Requests), release notes, and other assorted forms of documentation all in a searchable database. Generally, it's where I start to research a problem, and there is almost always some sniplet of info pertaining to my problem.
Otherwise, I haven't been to the OTN site, so I am not sure what documentation is there.