I've just come back, quite literally, I'm sat in the airport car park waiting for the aa as my car won't start.
I loved it! I had three full days, I did the golden circle tour, the south coast tour and a day wandering around reykjavik, cafes, shops, museums etc. I saw the northern lights,they were amazing, but out of the four nights I was there, the trip was cancelled three nights due to cloud. So I would see it as a nice bonus if you do manage to see them.
The weather was really mixed, Mon and Tues were freezing but bright and dry, yesterday was freezing, blizzards, wind like I've never experienced before. If you have ski clothes it would be worth taking them.
I stayed at the downtown youth hostel which was really well placed, much cheaper than a hotel and I had booked a double room with en suite, I actually had a triple room. Worth considering if you are on a budget.
I didn't manage to get to one, but the hostel staff recommended going to one of the city pools as a cheaper, more authentically, icelandic experience than the blue lagoon.
All in all, I would recommend iceland as a great short break destination. I'd love to go back in the summer, to go whale watching and see the sights without the snow!