I haven't been but many of my colleagues have, I'd really like to go though. Bear in mind everything is very expensive, and don't go for a 'traditional icelandik feast' unless you want to eat whale blubber and (not sushi) raw fish. There are many really good excursions, to the hot springs where you can bathe in the mud and to the geysers and glaciers.
I don't think its a great time for whale watching but there is a chance you may see some, particularly the minkes and smaller species.
(Also its a usual stopover enroute to the states )
My sister went to Iceland for her honeymoon. She loved it - not much to do though (not that you'd need lots of activities on your honeymoon ). She did visit the Blue Lagoon a couple of times.
She says Iceland was MEGA expensive.