As others have stated, this has become one of my favorite spots in Madison.
The happy hour is one of the best, and the specialty drinks are unique, delicious and potent. Getting them for half price from 4-6 is just icing on the cake. However, you will have to get there quite early to secure seating in the bar area since the happy hour special is only good in that side of the restaurant.
I have eaten both brunch and dinner at Jac's and have yet to be disappointed. I have tried a majority of the menu items, but my favorite still remains the mussels. They are simply the best in town. It is also great to have several vegetarian and many gluten free options so this location is always an option for those who have more restrictive diets.
Finally, the waitstaff and bartenders are both attentive and friendly regardless of how busy it gets.