Metalink is Oracle's web based support site...
Providing you have an active support contract with Oracle, you should be able to use your SAC to register at support.oracle.com (SAC = support Access code - used to raise Telephone support with Oracle)

I use Metalink all the time....

Whenever I'm doing something new it contains notes, froum questions etc. for me to get more of an idea of how things work and ways to implement them..

whenever I hit an error, I search metalink for it to find out workarounds, bugs.

Use it to raise Oracle support requests

download Software patches

Read alerts

I'm trying to think more of a situation when I wouldn't use it!