I've never eaten in Waco itself, but if you're in the area, you should drop in the Czech Stop, a bakery (which looks a lot more like a convenience store, really) just north of Waco off Exit 353 of I-35 (I believe the town is actually called West). They sell sandwiches and other such fare, but the real draw is the kolaches, which are these little Czech-style yeast roll with fillin's - sausage, jalapeno, cream cheese, strawberry, apricot, etc. You can find kolaches at many, many places just off the highway in central Texas, and for lots of people (myself included), they are a must for any road trip.
The Czech stop and its kolaches ain't fancy, but they're mighty tasty, and if you want to eat what the locals eat, they're the way to go.
I'm sure that others more familiar with the Waco area can also recommend some good barbecue. All of my smoked meat eating has been much closer to Austin or Houston, however.
"Make me one with everything."
-Zen master ordering a hot dog