Eternal Witness is the best Fifth Dawn card, the best creature in Standard, and probably the best green creature ever printed.
To drive the point home, Aaron Forsythe played Eternal Witness in his Oath of Druids deck at the SCG 5-proxy tourney. You know, an Oath deck, where you can pick any creature in Magic to play (except Phage and Phyrexian Dreadnaught and other types).
Eternal Witness alongside with Sakura-Tribe Elder makes a superb engine for any pseudo-control deck.
Eternal Witness combos with everything (and really stinking well with Astral Slide and Unearth), and it gets better and better the longer the game lasts. Her robustness allows you to replay all your Eternal Witnesses should you draw a fresh one.
She is a perfect Johnny/Spike card and you just can't ask for more than perfection.