I have been going to Pedro's for more or less ten years. I would come visit friends in the Madison area every year when I was living in Pennsylvania during high school, and a trip to Pedro's was always on the list - my friends were obsessed.
Pedro's isn't authentic Mexican fare, and I don't think anyone pretends it is. It used to be an affordable, tasty, Americanized Mexican restaurant with half-priced margaritas on T/Th, good chips and salsa, and delicious made-fresh tortillas (literally - they have a tortilla press in the dining room and they crank them out and bring them to your table while you're deciding what to order).
I was a big fan of Pedro's until the last year or so. They raised their prices while lowering their quality. The last time I was there, I got a shredded beef chimi with beans and rice. In former years, I could make 2-3 servings from the chimi and finish my sides at the restaurant if I was willing to eat to the point of discomfort (which I sometimes was, because it used to be good). This last time, there was about 1/3 of the amount of beans and rice that they used to serve, and the chimi was about half as big. Also, almost half of my filling for the chimichanga was two large chunks of unrendered fat. Gross. Gross gross gross. I don't know how the cooks could have missed it - it wasn't like a small little chunk a la pork 'n beans that got buried under the meat - it was meat on one side, fat on the other. Barf.
The food has been sub-par my last couple of visits as well. I won't totally write it off, as the margarita deal is a really good one, and the tortillas are delicious, so I'll just suggest it as a drink & snack stop, especially in the warmer months when you can sit on the patio.