Aigo Bistro, 84 Thoreau St. in Concord, MA, is possibly the best restaurant in the area. My wife and I have eaten there maybe 6 times over the last few years (we save this for special occasions), and we have never had anything but superb meals.
Although they tend to be on the pricy side (for Concord), Aigo Bistro is Highly Recommended.
Walden Grille, 24 Walden St., in Concord Center.
My wife and I ate dinner here on Saturday, May 17, 2003. It was a disaster. Even allowing 90 minutes for dinner before a show (across the street), our dinner arrived 10 minutes before we had to leave, and it was grossly overcooked to the point of being charred (tenderloin in puff pastry).
The manager was polite about not charging us for the inedible food, but the bad service plus the horrible preparation of the entree earns Walden Grille a rating of Not Recommended.