I guess it depends on what you want to do, and how long you're staying.  Traveling around the country might be a little more difficult, but if you're just staying in the Reykjavik area you should be fine. 

I just got back from a short trip to Reykjavik, and had a great time. As another commenter said, more darkness means more time for the Northern Lights. Also more time for drinking and partying, if that's your thing - Reykjavik has lively, non-stop night-life during the weekends.

There is also good food, and lots of public heated swimming pools to explore (my blog entry about my visit to some of those pools should be up in the next day or two).  Have fun!