This is one of the hidden "gems" in Icelandic fast food flora. It is especially popular late in the evening and should always be last stop of the pub crawl. It is really easy to make friends in the line late in the am.
By far one of the hidden treasures of Reykjavik fast food scene. I recommend that you go there for lunch early in your trip - as you just might end up wanting to go again and again! Open late during weekends, making it a good stop after few drinks at the bar.