Isaac's has been a favorite place of mine over all the years I lived in or near Newtown.
Their variety of beer is the main reason to go - they have always have a good rotating selection as well as a wide range of other available bottles and tap beers.
The food isn't bad, although I haven't tried everything on the menu, I can vouch for the Turkey Burger, the beef burgers are all great, and you can't go wrong with any of the appetizers - I especially like the brie or the spinach/artichoke dip.
It can get a bit crowded, so try to go early on weekends, or just be prepared to wait. They also won't seat you until your entire party is there which is kind of a pain, but just something to keep in mind.
The atmosphere is great and you'll love sitting outside on nice days. There's an upstairs with seating also, but that's usually where they put families, so if you're on a date or just out with friends, ask to be seated downstairs.