Probably one of my favorite bars in Madison. I have been here a couple times now with friends and have always had a wonderful time. The bar itself is wonderfully cozy but still has room for larger groups. Additionally, it's not too loud - you feel like the place has energy and is lively but you are still able to hear your friends talk.
They have a great selection of ever-changing beers on tap and they do a good job of bringing in beers from breweries that are lesser known but still delicious. Both times I've been there, it's been the same guy manning the bar and he is friendly, though I can tell he's often a little frazzled because the bar is pretty busy. The bar doesn't have food aside from the usual bar snacks but you are able to order pizza from Falbo Bros next door. My friends and I did this once night and the pizza was actually really yummy!
I have heard that they have great live music here but I am never able to stay long enough to hear it so I definitely will be back to experience live music at the Mason Lounge.