Lulu's is a winner. Situated nearby a slew of much-praised restaurants, this mom and pop place lives up to its location. The staff is knowledgable and friendly, and the place is sparse but clean. The ingredients they use are always fresh, and the flour tortillas and guacamole are especially high quality. I would recommend both the cheese chile relleno and the veggie burrito; they definitely keep me coming back for more. I've also been told by many friends that the beef at Lulu's is prepared very well.
For Guadalajara fare in the valley, I don't think I'd go anywhere else!