Purchasing the Hero's Pack nets you a special mount, costume, and apprentice. You also nab 10 Mithril (which can be sold for a mountain of in-game currency), 600 East Sector Treasures (used for summoning a boss), and a ring (used for wearing a ring and getting a stat boost). Players who purchase now will get early access to the open beta as well. If you're enjoying the closed test or just like the idea and really want to buy into the game now, the option is open.
[Source: Perfect World Entertainment press release]