We went there to celebrate my 60th but only for a week as the other week was spent on Islay (whisky tours)...
Iceland is an awesome place; we've been to many beautiful countries, yet I would go back there in a heartbeat. There is so much to see and experience that you must, must do your research and plan your visit to get the most out of it. Other caveats are obviously the weather - it really is the only country where we experienced sun, rain, snow, hail and gale force winds in one day; so, you do need appropriate all-weather gear, especially layers that you can remove and replace through the day. Include thermals on your list. It is a vast country so getting around takes time and you do need a 4 x 4 hire car as you will need to leave the main ring road to drive along tracks to see waterfalls, glaciers etc. You can see many wonderful sights by only touring along the south coast, as we did. Plan each journey to try and visit several places in a day. If you want to see the Aura Borealis you do need to go in late December or January (we went in October and disappointingly didn't see it). Finally, it is eye wateringly expensive so do save up!