Actually, the number of attractions open during the 8am early entry is somewhat limited. However, DEFINITELY do it - if not for the simple opportunity to ride Spiderman and Men in Black as much as you want without waiting.
The attractions open at IOA for early entry are:
Spiderman
Storm Force
Cat in the Hat
One Fish Two Fish
Carouseussel
The attractions open at USF for early entry are (and please correct me if I'm wrong - I'm more familiar with IOA):
Men in Black
Back to the Future
ET