Sardine is one of my favorite brunch spots in Madison. Besides the good food, the location is right off the lake and the restaurant has great ambiance. My go to is the smoked salmon sandwich that is served with mixed greens and frites, and of course a bloody mary on the side. If your a bloody mary drinker, expect this one to be a little different than your standard bloody, but it's still good and worth a try.
Brunch is usually busy here, but we've never had a problem or wait to really get a table for two. I recommend grabbing a seat at the bar if you can, which is not much different than sitting at one of the high tops anyway (plus you'll avoid any wait). If you're coming in a large group, I would definitely call in advance for reservations.
I don't think you can really go wrong with any of their menu items. People rave even about the burger on their menu. I've only had brunch here, so I can't comment on their dinner options.
The one thing that can be a challenge at times is parking, as Sardine is in Machinery Row which is a building shared by other businesses. You may find yourself patiently waiting for a spot to open up when its busy. :)