I was surprised to find that there have been no reviews on Arwen so far, as she seems to be a very popular - and powerful - hero. Well then, here we go.
You want some additional willpower? You need extra-resources every turn? You are looking for a reasonably cheap Spirit-hero without changing the rest of your deck? Enter the fair daughter of Elrond. With her 3 willpower she will probably concentrate on questing in most scenarios. Her combat-stats aren't spectacular, but if necessary she can block some smaller or medium-sized attacks as long as you can deal with unpredictable shadow-effects. Which shouldn't be a problem with Spirit (Hasty Stroke, Jubayr).
What really makes her shine, though, is her ability. Getting an additional resource every round from the beginning is more than helpful. Just think of all the good and expensive Spirit-allies (Northern Tracker, Elfhelm, Rhovanion Outrider) which tend to cost 3 or 4. Or remember how often you had to choose between playing a card or saving that one resource for A Test of Will. Why not enjoy the luxury of having both?
Note that she can give that resource to another (Noldor) character or to herself. She can even support her spouse Aragorn, but obviously she will often work with her father Elrond, who can pay for allies of any sphere.
Speaking of Noldor: in that archetype, discarding is no drawback but part of the fun. Think of Lords of the Eldar, The Evening Star or Glorfindel. The best card to complement her ability is, however, Elven-light, as you can switch between getting resources and cards just as the game-state requires.
Her only drawback is that you can't use the ally-version of her character (Arwen Undómiel). Apart from that, she seems to be one of the most versatile, straightforward and useful heroes in the game.
Finally, what I personally like about Arwen is that her mechanic somehow reflects the function of her character in the story of The Lord of the Rings, the book version, to be more precise. She is not the bold and active warrior-princess that is portrayed in the films. She's just that ever-present inspiration in the background for her people, her family and her Dunedain-love. Also consider the Tale of Tinúviel, ingame and story-wise.