Arlinn is a very aggressive planeswalker. Both of her Daybound and Nightbound sides have advantages, so it doesn't matter which time is on the table when you play her. During day you can create two 2/2 wolf tokens, and at night you can become a 5/5 wolf with a haste.
She could be a great addition to the Gruul Werewolves list with Tovolar, or she could accompany Goldspan Dragon in a more traditional Gruul Aggro deck. In either case, you will never regret paying 4 mana for her.