I have dined at Union Grill twice, both times for dinner with coworkers. I have found the service to be good and the food to be a good value (i.e. large tasty portions).
On trip #1 I had the fish sandwich, which was certainly hearty. 2 large pieces of fish (cod? perch?) deep-fried on a bun with tartar sauce and waffle fries on the side. The sandwich was filling and tasty, if a bit bland.
On trip #2 I had a cup of the chicken and sausage gumbo (very good) and the Pittsburgh-style chicken salad with the house dressing (a garlic vinaigrette). The salad was large, and the standouts were the sauteed mushrooms and onions, as well as the waffle fries (which I could probably eat for days). The chicken was bland and a little bit dry.
There are plenty of other items on the menu that I want to try (including the Turkey Devonshire) so I will surely return. The location is super convenient to CMU and Pitt, and drink specials look promising (although I haven't actually tried the Union Grill for drinks).