I had the chance to visit Iceland while on business in Boston for a week in March. The weather is still pretty darn cold, so bring your thermals, you will need them!
I have heard that the best time to go is Summer, but with the package deals that you can get in March to Iceland, I decided March would work. I did the usual tourist tours: The Northern Lights, The Geyser, The Blue Lagoon Spa, but the charm of Iceland is definitely the people. Very friendly ... I felt like I was invited and welcomed from the first step off the plane.
I did a quick weekend tour, but I recommend going in the Summer time and staying longer than 3 days! The history is rich and the language is the same as it was back in the Viking days. Very charming and welcoming!