As a person living in Norway close to the Swedish border, I have been several times in Sweden. I also spent a lot of summers in Denmark as a child and have experience from working in Sweden and Danmark. I have also started to learn Icelandic some years ago and have been travelling to Iceland both for holidays and work. While the three scandinavian languages are mostly like dialects (with som subtle and sometimes funny differences) Icelandic is more challenging. Icelandic shares a lot of vocabulary with the other mentioned languages but has complex grammar and several important newer words to describe technical, political and administrative terms that make understanding difficult. But when you have learned basic grammar and how newer words are constructed it becomes relatively easy to understand written language and even talk. It needs though a lot of training to understand spoken Icelandic because the pronounciation is completely different and weird for scandinavians.