Turns continue, alternating between players, until the end of the game
It gives all your creatures +1/+1, trample, and infect until end of turn
Legality depends on the tournament format -- Head of the Class can be legal in some formats and banned or restricted in others
Cards like Thalia, Guardian of Thraben can help you put one more color of mana into spells like Painful Truths, Radiant Flames, or Unified Front, provided you have additional colors to spend on the extra mana
Pokmon Center exclusive collectors
If youre someone (like me) who tries to pick up an ETB and then buys a bunch of loose packs here and there, youre now just shit out of luck