I've been living in the Madison area for about 6 years and in Verona for over 4. I've been to many of the Mexican restaurants in the area, including large, medium, and "hole in the wall".
I've now been to Cuco's around 7-8 times and believe it's one of the best Mexican restaurants in the Madison area. The service is always super fast, the atmosphere is decent, the food is great, and prices very reasonable. My wife and I also like their margaritas.
I'm often a creature of habit when it comes to restaurants, but I've had fun trying a variety of items at Cuco's, all have been great. I really like their Chile Rellanos. I also recommend trying a few of their specialties. For a twist on Fajita, I recommend trying their Molcajeta. I also really enjoyed their Burrito Cuco's.
If you're really hungry, the Especial Grande will do the trick...but don't be surprised when they bring out two large plates of food! All for around $12.