Achievements are the collection nobody finishes alone, because the hard ones need a coordinated group or a secret chain nobody solves twice. Glory of the Raider from Icecrown through Legion, Loremaster, The Insane, the Lucid Nightmare and Uuna secret chains and the Midnight season titles are all completed here.
Every achievement is earned on your character, which means the title, mount or pet attached to it is permanently yours.
Which achievements are worth buying?
The Glory meta-achievements are the best value, because each one carries a mount and needs a full group that knows every trick. Titles like The Insane are pure time — hundreds of hours of reputation — which is the other case where paying is rational.