You can take your coffee at the table or move to a small comfortable lounge to relax after your meal. The one thing you can't do is just show up and expect to be served on the spot. Reservations are a must. The place caters to groups and is a popular spot, especially in summer, for Icelandic weddings. They will accommodate you if they can.
Weather is also a factor since getting there entails a boat trip, albeit a short one.
The number is 852-0099.